dt-bindings: arm: Add compatible for MediaTek MT8186

This commit adds dt-binding documentation for the MediaTek MT8186
reference board.

Signed-off-by: Allen-KH Cheng <allen-kh.cheng@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220520122217.30716-3-allen-kh.cheng@mediatek.com


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