media: dt-bindings: rockchip: Document RK3399 Video Decoder bindings

Document the Rockchip RK3399 Video Decoder bindings.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>