arm: sunxi: Enable DM_MMC

Enable DM_MMC for all Allwinner SoCs, this will eventually
enable BLK.

Also removed DM_MMC enablement in few parts of sunxi
configurations.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>