spi: mt6779: update spi document

this patch update spi document for MT6779 SOC.
Signed-off-by: Mason Zhang <Mason.Zhang@mediatek.com>
Link: https://lore.kernel.org/r/20210226110109.30500-1-Mason.Zhang@mediatek.comSigned-off-by: Mark Brown <broonie@kernel.org>

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

@@ -13,6 +13,7 @@ Required properties:
     - mediatek,mt8183-spi: for mt8183 platforms
     - "mediatek,mt8192-spi", "mediatek,mt6765-spi": for mt8192 platforms
     - "mediatek,mt8516-spi", "mediatek,mt2712-spi": for mt8516 platforms
+    - "mediatek,mt6779-spi", "mediatek,mt6765-spi": for mt6779 platforms
 
 - #address-cells: should be 1.