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>

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

@@ -19,6 +19,8 @@ properties:
       - 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
@@ -29,6 +31,10 @@ properties:
   '#clock-cells':
     const: 1
 
+  mediatek,pnswap:
+    description: Invert polarity of the SGMII data lanes
+    type: boolean
+
 required:
   - compatible
   - reg