net-next: mediatek: document MediaTek SoC ethernet binding

This adds the binding documentation for the MediaTek Ethernet
controller.

Signed-off-by: John Crispin <blogic@openwrt.org>
Acked-by: Rob Herring <robh@kernel.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>