powerpc/corenet_ds: Document for the boot from SRIO

This document describes the implementation of the boot from SRIO,
includes the introduction of envionment, an example based on P4080DS
platform, an example of the slave's RCW, and the description about
how to use this feature.

Signed-off-by: Liu Gang <Gang.Liu@freescale.com>

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+------------------------------
+SRIO Boot on Corenet Platforms
+------------------------------
+
+For some PowerPC processors with SRIO interface, boot location can be configured
+to SRIO by RCW. The processor booting from SRIO can do without flash for u-boot
+image, ucode and ENV. All the images can be fetched from another processor's
+memory space by SRIO link connected between them.
+
+This document describes the processes based on an example implemented on P4080DS
+platforms and a RCW example with boot from SRIO configuration.
+
+Environment of the SRIO boot:
+       a) Master and slave can be SOCs in one board or SOCs in separate boards.
+       b) They are connected with SRIO links, whether 1x or 4x, and directly or
+          through switch system.
+       c) Only Master has NorFlash for booting, and all the Master's and Slave's
+          U-Boot images, UCodes will be stored in this flash.
+       d) Slave has its own EEPROM for RCW and PBI.
+       e) Slave's RCW should configure the SerDes for SRIO boot port, set the boot
+          location to SRIO, and holdoff all the cores if needed.
+
+       ----------        -----------             -----------
+       |                 |       |         |             |         |
+       |                 |       |         |             |         |
+       | NorFlash|<----->| Master  |    SRIO     |  Slave  |<---->[EEPROM]
+       |                 |       |         |<===========>|         |
+       |                 |       |         |             |         |
+       ----------        -----------             -----------
+
+The example based on P4080DS platform:
+       Two P4080DS platforms can be used to implement the boot from SRIO. Their SRIO
+       ports 0 will be connected directly and will be used for the boot from SRIO.
+
+       1. Slave's RCW example for boot from SRIO port 0 and core 0 not in holdoff.
+               00000000: aa55 aa55 010e 0100 0c58 0000 0000 0000
+               00000010: 1818 1818 0000 8888 7440 4000 0000 2000
+               00000020: f400 0000 0100 0000 0000 0000 0000 0000
+               00000030: 0000 0000 0083 0000 0000 0000 0000 0000
+               00000040: 0000 0000 0000 0000 0813 8040 698b 93fe
+
+       2. Slave's RCW example for boot from SRIO port 0 and all cores in holdoff.
+               00000000: aa55 aa55 010e 0100 0c58 0000 0000 0000
+               00000010: 1818 1818 0000 8888 7440 4000 0000 2000
+               00000020: f440 0000 0100 0000 0000 0000 0000 0000
+               00000030: 0000 0000 0083 0000 0000 0000 0000 0000
+               00000040: 0000 0000 0000 0000 0813 8040 063c 778f
+
+       3. Sequence in Step by Step.
+               a) Update RCW for slave with boot from SRIO port 0 configuration.
+               b) Program slave's U-Boot image, UCode, and ENV parameters into master's
+                  NorFlash.
+               c) Start up master and it will boot up normally from its NorFlash.
+                  Then, it will finish necessary configurations for slave's boot from
+                  SRIO port 0.
+               d) Master will set inbound SRIO windows covered slave's U-Boot image stored
+                  in master's NorFlash.
+               e) Master will set an inbound SRIO window covered slave's UCode stored in
+                  master's NorFlash.
+               f) Master will set an inbound SRIO window covered slave's ENV stored in
+                  master's NorFlash.
+               g) If need to release slave's core, master will set outbound SRIO windows
+                  in order to configure slave's registers for the core's releasing.
+               h) If all cores of slave in holdoff, slave should be powered on before all
+                  the above master's steps, and wait to be released by master. If not all
+                  cores in holdoff, that means core 0 will start up normally, slave should
+                  be powered on after all the above master's steps. In the startup phase
+                  of the slave from SRIO, it will finish some necessary configurations.
+               i) Slave will set a specific TLB entry for the boot process.
+               j) Slave will set a LAW entry with the TargetID SRIO port 0 for the boot.
+               k) Slave will set a specific TLB entry in order to fetch UCode and ENV
+                  from master.
+               l) Slave will set a LAW entry with the TargetID SRIO port 0 for UCode and ENV.
+
+How to use this feature:
+       To use this feature, you need to focus three points.
+
+       1. Slave's RCW with SRIO boot configurations, and all cores in holdoff
+          configurations if needed.
+          Please refer to the examples given above.
+
+       2. U-Boot image's compilation.
+               For master, U-Boot image should be generated specifically by
+
+                               make xxxx_SRIOBOOT_MASTER_config.
+
+               For example, master U-Boot image used on P4080DS should be compiled with
+
+                               make P4080DS_SRIOBOOT_MASTER_config.
+
+               For slave, U-Boot image should be generated specifically by
+
+                               make xxxx_SRIOBOOT_SLAVE_config.
+
+               For example, slave U-Boot image used on P4080DS should be compiled with
+
+                               make P4080DS_SRIOBOOT_SLAVE_config.
+
+       3. Necessary modifications based on a specific environment.
+               For a specific environment, the SRIO port for boot, the addresses of the
+               slave's U-Boot image, UCode, ENV stored in master's NorFlash, and any other
+               configurations can be modified in the file:
+                                       include/configs/corenet_ds.h.