arm64: allwinner: pine64: add MMC support

All Pine64 boards connect an micro-SD card slot to the first MMC
controller.
Enable the respective DT node and specify the (always-on) regulator
and card-detect pin.
As a micro-SD slot does not feature a write-protect switch, we disable
this feature.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>