media: dt-bindings: Add binding for rk3228 rga

Indicate that the rk3228 rga is compatible with that of the rk3288.

But if any rk3228-specific quirks are identified in future that
require handling logic that differs from what is provided for the
rk3288, then allow for the compatibility string "rockchip,rk3228-rga"
to be matched instead of "rockchip,rk3288-rga".

Signed-off-by: Justin Swartz <justin.swartz@risingedge.co.za>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/devicetree/bindings/media/rockchip-rga.txt

@@ -6,8 +6,9 @@ BitBLT, alpha blending and image blur/sharpness.
 
 Required properties:
 - compatible: value should be one of the following
-		"rockchip,rk3288-rga";
-		"rockchip,rk3399-rga";
+  "rockchip,rk3228-rga", "rockchip,rk3288-rga": for Rockchip RK3228
+  "rockchip,rk3288-rga": for Rockchip RK3288
+  "rockchip,rk3399-rga": for Rockchip RK3399
 
 - interrupts: RGA interrupt specifier.