dfu: Add proper dependency for CONFIG_DFU_MMC

When CONFIG_CMD_MMC and CONFIG_MMC are disabled, still some compilation
errors are seen as below due to unresolved symbols.

drivers/dfu/dfu_mmc.o: in function `mmc_block_op':
drivers/dfu/dfu_mmc.c:32: undefined reference to `find_mmc_device'
drivers/dfu/dfu_mmc.c:54: undefined reference to `mmc_get_blk_desc'
drivers/dfu/dfu_mmc.c:67: undefined reference to `mmc_get_blk_desc'
drivers/dfu/dfu_mmc.c:70: undefined reference to `mmc_get_blk_desc'
drivers/dfu/dfu_mmc.o: in function `dfu_fill_entity_mmc':
drivers/dfu/dfu_mmc.c:369: undefined reference to `find_mmc_device'
drivers/dfu/dfu_mmc.c:376: undefined reference to `mmc_init'
drivers/dfu/dfu_mmc.c:403: undefined reference to `mmc_get_blk_desc'
gnu/aarch64/lin/aarch64-linux/bin/aarch64-linux-gnu-ld.bfd: line 4:
31661 Segmentation fault      (core dumped) $CC --sysroot=$LIBC
--no-warn-rwx-segment "$@"
Makefile:1760: recipe for target 'u-boot' failed
make: *** [u-boot] Error 139
make: *** Deleting file 'u-boot'

Add dependency of CONFIG_MMC for CONFIG_DFU_MMC config to fix the errors.

Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@amd.com>