arm64: zynqmp: spl: install a PMU firmware config object at runtime

arm64: zynqmp: spl: install a PMU firmware config object at runtime

Optionally allow U-Boot to load a configuration object into the Power
Management Unit (PMU) firmware on Xilinx ZynqMP.

The configuration object is required by the PMU FW to enable most SoC
peripherals. So far the only way to boot using U-Boot SPL was to hard-code
the configuration object in the PMU firmware. Allow a different boot
process, where the PMU FW is equal for any ZynqMP chip and its
configuration is passed at runtime by U-Boot SPL.

All the code for Inter-processor communication with the PMU is isolated in
a new file (pmu_ipc.c). The code is inspired by the same feature as
implemented in the Xilinx First Stage Bootloader (FSBL) and Arm Trusted
Firmware:

 * https://github.com/Xilinx/embeddedsw/blob/fb647e6b4c00f5154eba52a88b948195b6f1dc2b/lib/sw_apps/zynqmp_fsbl/src/xfsbl_misc_drivers.c#L295
 * https://github.com/ARM-software/arm-trusted-firmware/blob/c48d02bade88b07fa7f43aa44e5217f68e5d047f/plat/xilinx/zynqmp/pm_service/pm_api_sys.c#L357

SPL logs on the console before loading the configuration object:

  U-Boot SPL 2019.07-rc1-00511-gaec224515c87 (May 15 2019 - 08:43:41 +0200)
  Loading PMUFW cfg obj (2008 bytes)
  EL Level:	EL3
  ...

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>