sunxi: mmc: remove usage of struct sunxi_ccm_reg

sunxi: mmc: remove usage of struct sunxi_ccm_reg

The Allwinner MMC code uses a complex C struct, modelling the clock
device's register frame. We rely on sharing the member names across all
Allwinner SoCs, which is fragile.

Drop the usage of the struct in the MMC code, by using #define'd
register names and their offset, and then adding those names to the base
pointer. This requires to define those offsets for all SoCs, but since we
only use between four and six clock registers in the MMC code, this is
easily done.

This removes one common user of the clock register struct.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>