spi: mediatek: update spi master bingdings for MT6893 SOC

this patch update spi master bingdings for MT6893 SOC.
Signed-off-by: Mason Zhang <Mason.Zhang@mediatek.com>
Link: https://lore.kernel.org/r/20210629101310.21045-1-mason.zhang@mediatek.comSigned-off-by: Mark Brown <broonie@kernel.org>

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

@@ -11,6 +11,7 @@ Required properties:
     - mediatek,mt8135-spi: for mt8135 platforms
     - mediatek,mt8173-spi: for mt8173 platforms
     - mediatek,mt8183-spi: for mt8183 platforms
+    - mediatek,mt6893-spi: for mt6893 platforms
     - "mediatek,mt8192-spi", "mediatek,mt6765-spi": for mt8192 platforms
     - "mediatek,mt8195-spi", "mediatek,mt6765-spi": for mt8195 platforms
     - "mediatek,mt8516-spi", "mediatek,mt2712-spi": for mt8516 platforms