DTS: serial: Add bindings document for the Mediatek UARTs

This patch add s devicetree document for Mediatek UART.
Signed-off-by: Eddie Huang <eddie.huang@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>

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

@@ -2,6 +2,8 @@
 
 Required properties:
 - compatible should contain:
+  * "mediatek,mt8135-uart" for MT8135 compatible UARTS
+  * "mediatek,mt8127-uart" for MT8127 compatible UARTS
   * "mediatek,mt6589-uart" for MT6589 compatible UARTS
   * "mediatek,mt6582-uart" for MT6582 compatible UARTS
   * "mediatek,mt6577-uart" for all compatible UARTS (MT6589, MT6582, MT6577)