ARM: keystone: cmd_ddr3: add ddr3 commands to test ddr

ARM: keystone: cmd_ddr3: add ddr3 commands to test ddr

Add ddr3 commands:

test <start_addr in hex> <end_addr in hex> - test DDR from start\n
	address to end address\n
ddr compare <start_addr in hex> <end_addr in hex> <size in hex> -\n
	compare DDR data of (size) bytes from start address to end
	address\n
ddr ecc_err <addr in hex> <bit_err in hex> - generate bit errors\n
	in DDR data at <addr>, the command will read a 32-bit data\n
	from <addr>, and write (data ^ bit_err) back to <addr>\n

Delete CONFIG_MAX_UBOOT_MEM_SIZE, as it was supposed to be used
for ddr3 commands and for now it's not needed any more.

Signed-off-by: Hao Zhang <hzhang@ti.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>