media: dt-bindings: media: mtk-vcodec: document SCP node

The mediatek codecs can use either the VPU or the SCP as their interface
to firmware. Reflect this in the DT bindings.

Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Acked-by: Tiffany Lin <tiffany.lin@mediatek.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>