dt-bindings: i2c-mtk: Add mt7623 binding

The mt7623 dtsi has support for the i2c block, but this is not documented.
Add the documentation for SoC mt7623 to de description.
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Wolfram Sang <wsa@the-dreams.de>

Documentation/devicetree/bindings/i2c/i2c-mtk.txt

@@ -6,6 +6,7 @@ Required properties:
   - compatible: value should be either of the following.
       "mediatek,mt6577-i2c": for i2c compatible with mt6577.
       "mediatek,mt6589-i2c": for i2c compatible with mt6589.
+      "mediatek,mt7623-i2c", "mediatek,mt6577-i2c": for i2c compatible with mt7623.
       "mediatek,mt8173-i2c": for i2c compatible with mt8173.
   - reg: physical base address of the controller and dma base, length of memory
     mapped region.