[U-Boot] disabling mmc in spl when booting using bootrom

[Sandy Patterson]

Simon,

I am trying to format a patch to disable MMC in the SPL if booting main
u-boot using BOOTROM, therefore the SPL MMC isn't needed.

Is the best solution to wrap every header file (rock2.h firefly-rk3288.h,
etc) with ifdefs on the BACK_TO_BROM define? Or would it be better to move
the SPL MMC define into rk3288-common.h and just have chromebook_jerry
undef it like it does the SPL GPIO code.

With that change, enabling BOOT_TO_BROM shrinks the spl from 32K to 23K.

Sandy