media: dt-bindings: Document the Rockchip VPU bindings

Add devicetree binding documentation for Rockchip Video Processing
Unit IP.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>