[media] dt-bindings: Add a binding for Mediatek Video Encoder

Add a DT binding documentation of Video Encoder for the
MT8173 SoC from Mediatek.

Signed-off-by: Tiffany Lin <tiffany.lin@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>