dt-binding: serial: mediatek,uart: update bingding for MT8188

Add a DT binding documentation for the MT8188 soc.

Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kewei Xu <kewei.xu@mediatek.com>
Link: https://lore.kernel.org/r/20220809084457.31381-1-kewei.xu@mediatek.com


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