media: dt-bindings: media: mtk-vcodec: Separating mtk vcodec encoder node

Updates binding document since the avc and vp8 hardware encoder in
MT8173 are now separated. Separate "mediatek,mt8173-vcodec-enc" to
"mediatek,mt8173-vcodec-enc-vp8" and "mediatek,mt8173-vcodec-enc".

This patch is not a compatible change, but we must do this modifaction
because MediaTek IOMMU add the device_link between the smi-larb
device and venc_device, if the venc device call the pm_runtime_get_sync,
the smi-larb's pm_runtime_get_sync also be called automatically.

There is a bit of backward compatibility for avc encoder, the avc
encoder device node still has compatible "mediatek,mt8173-vcodec-enc".

Acked-by: Tiffany Lin <tiffany.lin@mediatek.com>
Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Signed-off-by: Maoguang Meng <maoguang.meng@mediatek.com>
Signed-off-by: Irui Wang <irui.wang@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+huawei@kernel.org>