cmd: CONFIG_CMD_MMC depends on CONFIG_MMC

Trying to compile with CONFIG_CMD_MMC=y and CONFIG_MMC=n leads to errors:

riscv64-linux-gnu-ld.bfd: cmd/built-in.o: in function `do_mmcops':
cmd/mmc.c:984: undefined reference to `get_mmc_num'
riscv64-linux-gnu-ld.bfd: cmd/built-in.o: in function `do_mmc_setdsr':
cmd/mmc.c:873: undefined reference to `find_mmc_device'

Add missing dependency.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>