dt-bindings: spi: update bindings for MT6765 SoC

Add a DT binding documentation for the MT6765 soc.
Signed-off-by: luhua.xu <luhua.xu@mediatek.com>
Link: https://lore.kernel.org/r/1568195731-3239-2-git-send-email-luhua.xu@mediatek.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-mt65xx.txt

@@ -5,6 +5,7 @@ Required properties:
     - mediatek,mt2701-spi: for mt2701 platforms
     - mediatek,mt2712-spi: for mt2712 platforms
     - mediatek,mt6589-spi: for mt6589 platforms
+    - mediatek,mt6765-spi: for mt6765 platforms
     - mediatek,mt7622-spi: for mt7622 platforms
     - "mediatek,mt7629-spi", "mediatek,mt7622-spi": for mt7629 platforms
     - mediatek,mt8135-spi: for mt8135 platforms