dt-bindings: net: pcs: mediatek,sgmiisys: add MT7981 SoC

Add mediatek,pnswap boolean property needed on many boards using the
MediaTek MT7981 SoC.

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