arm64: dts: meson: g12a: add mdio multiplexer

Add the g12a mdio multiplexer which allows to connect to either
an external phy through the SoC pins or the internal 10/100 phy

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