dt-bindings: net: mediatek,net: add mt7986-eth binding

Introduce dts bindings for mt7986 soc in mediatek,net.yaml.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>