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.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>