ARM: dts: sun6i: Add mmc controller nodes

Add nodes for the 4 mmc controllers found on A31 SoCs to
arch/arm/boot/dts/sun6i-a31.dtsi.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>