powerpc: spiflash:Add corenet devices support in eSPI SPL

Existing eSPI SPL framework assumes booting from spi-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: York Sun <yorksun@freescale.com>