dt-bindings: serial: mtk-uart: add support for MT8516

Add binding documentation of mtk-uart for MT8516 SoC.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>