ARM64: dts: meson-gxbb-odroidc2: add the ethernet PHY's reset GPIO

This resets the ethernet PHY during boot to get the PHY into a "clean"
state. While here also explicitly specify the phy-mode instead of
relying on the default-value from meson-gx.dtsi.

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