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 <conor.dooley@microchip.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Sjoerd Simons <sjoerd@collabora.com>
Link: https://patch.msgid.link/20251115-openwrt-one-network-v4-3-48cbda2969ac@collabora.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

Author: sjoerdsimons

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

@@ -80,6 +80,7 @@ properties:
               - mediatek,mt2712-tphy
               - mediatek,mt6893-tphy
               - mediatek,mt7629-tphy
+              - mediatek,mt7981-tphy
               - mediatek,mt7986-tphy
               - mediatek,mt8183-tphy
               - mediatek,mt8186-tphy