dt-bindings: arm: Add compatible for Mediatek MT8195

This commit adds dt-binding documentation for the Mediatek MT8195
reference board.

Signed-off-by: Seiya Wang <seiya.wang@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210319023427.16711-8-seiya.wang@mediatek.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>