ata: add the readme for SATA command line

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

diff --git a/doc/README.sata b/doc/README.sata
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+1. SATA usage in U-boot
+
+       Two ways to operate the hard disk
+
+       * Raw read/write block from/to SATA hard disk;
+       * ext2load read file from ext2 filesytem in hard disk
+
+1.0 How to know the information of SATA hard disk?
+
+       => sata info
+
+SATA device 0: Model: ST3320620AS Firm: 3.AAD Ser#:             4QF01ZTN
+            Type: Hard Disk
+            Supports 48-bit addressing
+            Capacity: 305245.3 MB = 298.0 GB (625142448 x 512)
+
+1.1 How to save the kernel, filesystem, dtb to SATA hard disk with raw?
+
+       Notes: 0x1000 sectors = 2 MBytes
+
+       wirte kernel
+       => tftp 40000 /tftpboot/uImage.837x
+       => sata write 40000 0 2000
+
+       write ramdisk
+       => tftp 40000 /tftpboot/ramdisk.837x
+       => sata write 40000 2000 8000
+
+       write dtb
+       => tftp 40000 /tftpboot/mpc837xemds.dtb
+       => sata write 40000 a000 1000
+
+1.2 How to read the kernel, filesystem, dtb from SATA hard disk with raw?
+
+       load kernel
+       => sata read 200000 0 2000
+
+       load ramdisk
+       => sata read 1000000 2000 8000
+
+       load dtb
+       => sata read 2000000 a000 1000
+
+       boot
+       => bootm 200000 1000000 2000000
+
+1.3 How to load image from ext2 filesystem in U-boot?
+
+       U-boot doesn't support ext2 write to hard disk, so
+       you have to write the image to hard disk under Linux env,
+        before you load image from ext2 filesystem.
+
+       => ext2ls sata 0:1 /
+       <DIR>       4096 .
+       <DIR>       4096 ..
+       <DIR>      16384 lost+found
+                1352023 uImage.837x
+                3646377 ramdisk.837x
+                  12288 mpc837xemds.dtb
+                     12 hello.txt
+
+       => ext2load sata 0:1 200000 /uImage.837x
+
+       => ext2load sata 0:1 1000000 /ramdisk.837x
+
+       => ext2load sata 0:1 2000000 /mpc837xemds.dtb
+
+       => bootm 200000 1000000 2000000