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>