dt-bindings: mtk-uart: add mt6779 uart bindings

Add documentation for mt6779 uart dt-bindings
Signed-off-by: Mars Cheng <mars.cheng@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>

Documentation/devicetree/bindings/serial/mtk-uart.txt

@@ -9,6 +9,7 @@ Required properties:
   * "mediatek,mt6589-uart" for MT6589 compatible UARTS
   * "mediatek,mt6755-uart" for MT6755 compatible UARTS
   * "mediatek,mt6765-uart" for MT6765 compatible UARTS
+  * "mediatek,mt6779-uart" for MT6779 compatible UARTS
   * "mediatek,mt6795-uart" for MT6795 compatible UARTS
   * "mediatek,mt6797-uart" for MT6797 compatible UARTS
   * "mediatek,mt7622-uart" for MT7622 compatible UARTS