Add a DT binding documentation for the MT8195 soc.

Signed-off-by: Kewei.Xu <kewei.xu@mediatek.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>