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 <[e-mail]>
Signed-off-by: Daniel Golle <[e-mail]>
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/devicetree/bindings/net/pcs/mediatek,sgmiisys.yaml

···
       - enum:
           - mediatek,mt7622-sgmiisys
           - mediatek,mt7629-sgmiisys
+          - mediatek,mt7981-sgmiisys_0
+          - mediatek,mt7981-sgmiisys_1
           - mediatek,mt7986-sgmiisys_0
           - mediatek,mt7986-sgmiisys_1
       - const: syscon
···
 
   '#clock-cells':
     const: 1
+
+  mediatek,pnswap:
+    description: Invert polarity of the SGMII data lanes
+    type: boolean
 
 required:
   - compatible