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>

Documentation/devicetree/bindings/media/mediatek-vcodec.txt

@@ -19,7 +19,9 @@ Required properties:
 - iommus : should point to the respective IOMMU block with master port as
   argument, see Documentation/devicetree/bindings/iommu/mediatek,iommu.txt
   for details.
-- mediatek,vpu : the node of video processor unit
+One of the two following nodes:
+- mediatek,vpu : the node of the video processor unit, if using VPU.
+- mediatek,scp : the node of the SCP unit, if using SCP.
 
 
 Example: