dt-bindings: serial: Add compatible for Mediatek MT8186

This commit adds dt-binding documentation of uart for Mediatek MT8186 SoC
Platform.
Signed-off-by: Allen-KH Cheng <Allen-KH.Cheng@mediatek.com>
Link: https://lore.kernel.org/r/20220128062050.23978-3-allen-kh.cheng@mediatek.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -20,6 +20,7 @@ Required properties:
   * "mediatek,mt8135-uart" for MT8135 compatible UARTS
   * "mediatek,mt8173-uart" for MT8173 compatible UARTS
   * "mediatek,mt8183-uart", "mediatek,mt6577-uart" for MT8183 compatible UARTS
+  * "mediatek,mt8186-uart", "mediatek,mt6577-uart" for MT8183 compatible UARTS
   * "mediatek,mt8192-uart", "mediatek,mt6577-uart" for MT8192 compatible UARTS
   * "mediatek,mt8195-uart", "mediatek,mt6577-uart" for MT8195 compatible UARTS
   * "mediatek,mt8516-uart" for MT8516 compatible UARTS