The absence of this binding appears to be harmless in Linux but it breaks
Ethernet support in mainline u-boot. So add the binding (which is present
in all other u-boot supported GXBB device-trees).

Fixes: fb72c03e ("ARM64: dts: meson-gxbb-wetek: add a wetek specific dtsi to cleanup hub and play2")
Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lore.kernel.org/r/20211012052522.30873-3-christianshewitt@gmail.com