mmc: sunxi: Add DM_MMC support for H6

Unlike other Allwinner SoC's, H6 uses a different MMC mod clock offset.
Connect that with the respective compatible string.

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>