arm64: allwinner: a64: Add MMC pinctrl nodes

The A64 only has a single set of pins for each MMC controller. Since we
already have boards that require all of them, let's add them to the DTSI.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Tested-by: Florian Vaussard <florian.vaussard@heig-vd.ch>
Acked-by: Chen-Yu Tsai <wens@csie.org>