mmc: spl: select SPL_BLK for SPL_DM_MMC

mmc_bind() in mmc-uclass.c calls blk_create_devicef() which is
defined in blk-uclass.c, so SPL_BLK is required by SPL_DM_MMC.
Implicitly select SPL_BLK for SPL_DM_MMC.

Signed-off-by: Oleksandr Suvorov <oleksandr.suvorov@foundries.io>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>