spi: mediatek: Add bindings for mediatek MT2712 soc platform

This patch adds a DT binding documentation for the MT2712 soc.
Signed-off-by: Leilk Liu <leilk.liu@mediatek.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

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

@@ -3,6 +3,7 @@ Binding for MTK SPI controller
 Required properties:
 - compatible: should be one of the following.
     - mediatek,mt2701-spi: for mt2701 platforms
+    - mediatek,mt2712-spi: for mt2712 platforms
     - mediatek,mt6589-spi: for mt6589 platforms
     - mediatek,mt7622-spi: for mt7622 platforms
     - mediatek,mt8135-spi: for mt8135 platforms