dt-bindings: serial: mediatek,uart: add MT7981

Add compatible string for serial on MT7981 SoC.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240727121447.1016-1-zajec5@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/mediatek,uart.yaml

@@ -36,6 +36,7 @@ properties:
               - mediatek,mt7622-uart
               - mediatek,mt7623-uart
               - mediatek,mt7629-uart
+              - mediatek,mt7981-uart
               - mediatek,mt7986-uart
               - mediatek,mt7988-uart
               - mediatek,mt8127-uart