mmc: Add generic Kconfig option

Add a generic Kconfig option for the CONFIG_MMC option that was used before
in the configuration headers.

Since all the architectures need to be converted to that first, depend on
an non-existent config option that will be extended with architectures that
use that option.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>