dt-bindings: phy: mediatek,tphy: Add support for MT7981

Add a compatible string for Filogic 820, this chip integrates a MediaTek
generic T-PHY version 2

Acked-by: Conor Dooley <[e-mail]>
Reviewed-by: AngeloGioacchino Del Regno <[e-mail]>
Signed-off-by: Sjoerd Simons <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Vinod Koul <[e-mail]>

Documentation/devicetree/bindings/phy/mediatek,tphy.yaml

···
               - mediatek,mt2712-tphy
               - mediatek,mt6893-tphy
               - mediatek,mt7629-tphy
+              - mediatek,mt7981-tphy
               - mediatek,mt7986-tphy
               - mediatek,mt8183-tphy
               - mediatek,mt8186-tphy