ata: add the readme for SATA command line

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

doc/README.sata

+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