MMC is not initialized in SPL, so it cannot load u-boot.img
preventing boot from MMC.

Also driver specific functions are guarded with generic
configuration options which leads to build failures when device
driver is not enabled in config. Fix that by using driver
specific defines.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Acked-by: Stefano Babic <sbabic@denx.de>