DM: DM_MMC migration is now mandatory for non-SPL

As it has been now two years past the migration deadline, it is required
to have migrated.  Remove the check from the Makefile and rework some of
the Kconfig logic slightly to get the functional dependencies of DM_MMC
/ BLK right in both the SPL and non-SPL case.

Signed-off-by: Tom Rini <trini@konsulko.com>