dt-bindings: serial: Add compatible for Mediatek MT8192

This commit adds dt-binding documentation of uart for Mediatek MT8192 SoC
Platform.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Seiya Wang <seiya.wang@mediatek.com>
Link: https://lore.kernel.org/r/20200814060454.32200-3-seiya.wang@mediatek.com


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