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>