It is not possible to compile MMC/SPI SPL if the respective DM_MMC/DM_SPI
bits are not enabled. Secure the code with an ifdef to prevent compiler
splat.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Cc: Chin Liang See <clsee@altera.com>