ARM: dts: meson: add the SDIO MMC controller

Meson6, Meson8 and Meson8b are using the same MMC controller IP. This
adds the MMC controller node to meson.dtsi so it can be used by all
SoCs.

The controller itself is a bit special, because it has multiple slots.
Each slot is accessed through a sub-node of the controller. However,
currently the driver for this hardware only supports one slot.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>