dt-bindings: arm: Add compatible for Mediatek MT8192

This commit adds dt-binding documentation for the Mediatek MT8192
reference board.

Signed-off-by: Allen-KH Cheng <allen-kh.cheng@mediatek.com>
Link: https://lore.kernel.org/r/20220419092030.30519-1-allen-kh.cheng@mediatek.com
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>