blanche_defconfig: enable CONFIG_MMC

As commit 54925327 ("mmc: move CONFIG_GENERIC_MMC to Kconfig")
addressed, this is one of the last weird defconfigs that define
CONFIG_GENERIC_MMC without CONFIG_MMC.

Now I took a closer look at this.  Given that both CONFIG_CMD_MMC
and CONFIG_GENERIC_MMC are set for this defconfig, CONFIG_MMC should
be enabled.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>