ARM: dts: sun7i: Add mmc controller nodes

Add nodes for the 4 mmc controllers found on A20 SoCs to
arch/arm/boot/dts/sun7i-a20.dtsi.

Signed-off-by: David Lanzendörfer <david.lanzendoerfer@o2s.ch>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>