dt-bindings: serial: Add compatible for Mediatek MT8183

This adds dt-binding documentation of uart for Mediatek MT8183 SoC
Platform.

Signed-off-by: Erin Lo <erin.lo@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>