mmc: replace CONFIG_GENERIC_MMC with CONFIG_MMC

Now CONFIG_GENERIC_MMC and CONFIG_MMC match for all defconfig.
We do not need two options for the same feature.  Deprecate the
former.

This commit was generated with the sed script 's/GENERIC_MMC/MMC/'
and manual fixup of drivers/mmc/Kconfig.

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