dt-bindings: media: rockchip-rga: add rockchip,rk3588-rga

Add a new compatible for the rk3588 Rockchip SoC, which also features an
RGA, which is called RGA2 in the TRM Part2. It is the same core as used
on the rk3288 and rk3568, which documents the same RGA2.

Signed-off-by: Jianfeng Liu <liujianfeng1994@gmail.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>