ARM64: dts: meson-gxl-p23x: Enable ethernet

Enable Ethernet on the p23x board, pinctrl attribute is only added for
the p230 board since the p231 only uses the Internal PHY.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>