powerpc: mmc: Add corenet devices support in esdhc spl

powerpc: mmc: Add corenet devices support in esdhc spl

Existing eSDHC SPL framework assumes booting from sd-image
with boot_format header which contains final u-boot Image
offset and size. No such header is present in case of
corenet devices like T1040 as corenet deivces use PBI-RCW
based intialization.

So, for corenet deives, SPL bootloader use values provided
at compilation time. These values can be defined in board
specific config file.

Signed-off-by: Priyanka Jain <Priyanka.Jain@freescale.com>
Acked-by: Pantelis Antoniou <panto@antoniou-consulting.com>