ARM64: dts: meson-gx: remove the phy-mode property from meson-gx

The ethmac node has to be configured for each board due to different
pinctrl nodes for RGMII/RMII. Thus the phy-mode should be specified at
the same place (= in the board .dts), making it easier to read the board
.dts file (because the phy-mode is stated explicitly, without requiring
developers to read all "parent" .dtsi as well).

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