dt-bindings: serial: Add compatible for Mediatek MT8195

This commit adds dt-binding documentation of uart for Mediatek MT8195 SoC
Platform.

Signed-off-by: Seiya Wang <seiya.wang@mediatek.com>
Link: https://lore.kernel.org/r/20210319023427.16711-4-seiya.wang@mediatek.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>