media: dt-bindings: media: mtk-vcodec: Add binding for MT8192 VENC

Updates binding document for mt8192 encoder driver.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Irui Wang <irui.wang@mediatek.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>