mmc: sunxi: Refactor mod clock register offset

So far the only difference between the various Allwinner MMC controller
we are concerned about is the mod clock register offset.
This is actually not directly related to the MMC controller IP, but an
integration choice, dependent on the SoC this appears in.

To avoid becoming trapped with some compatible fallback strings, let's
remove the whole struct sunxi_mmc_variant, and replace this with a SoC
based choice, which we can derive from the CONFIG_MACH_SUNx_y symbols.

This will later simplify H616 support.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>