drivers/mmc/Kconfig: Make DM_MMC a hidden symbol

drivers/mmc/Kconfig: Make DM_MMC a hidden symbol

At this point in time, DM is always enabled. So if MMC is enabled, it
should select DM_MMC. No drivers need to depend on DM_MMC being enabled
now, so remove that from dependency lists. This now means that a number
of platforms which select'd DM_MMC need to select MMC instead. This also
fixes a migration problem with espresso7420 in that MMC is built again
with the platform.

Reviewed-by: Peter Robinson <pbrobinson@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>