dt-bindings: net: mediatek,net: add mt7981-eth binding

Introduce DT bindings for the MT7981 SoC to mediatek,net.yaml.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>