mmc: sunxi: Add DM clk and reset support

Now that we have the gate clocks and the reset gates in our new
Allwinner clock driver, let's make use of them in the MMC driver, when
DM_MMC is defined.
We treat the reset device as optional now, as the older SoCs don't
implement it.

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