media: dt-bindings: media: mtk-vcodec: Separate video encoder and decoder dt-bindings

Separate decoder and encoder document for the dts are big difference.

Signed-off-by: Yunfei Dong <yunfei.dong@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>