spi: dt-bindings: mediatek,spi-mt65xx: add compatible for MT7981

MT7981 has SPI controllers based on IPM design
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://patch.msgid.link/20240727114828.29558-1-zajec5@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/mediatek,spi-mt65xx.yaml

@@ -33,6 +33,7 @@ properties:
           - const: mediatek,mt6765-spi
       - items:
           - enum:
+              - mediatek,mt7981-spi-ipm
               - mediatek,mt7986-spi-ipm
               - mediatek,mt8188-spi-ipm
           - const: mediatek,spi-ipm