[media] dt-bindings: Document the Rockchip RGA bindings

Add DT bindings documentation for Rockchip RGA

Signed-off-by: Jacob Chen <jacob-chen@iotwrt.com>
Signed-off-by: Yakir Yang <ykk@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>