ARM64: dts: meson-gxbb-p200: 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 specify the phy-handle of the ethmac node to make the
PHY configuration similar to the one we have on GXL devices. This will
allow us to specify OF-properties for the PHY itself.

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>