[media] MAINTAINERS: add entry for Rockchip RGA driver

Add MAINTAINERS entry for the rockchip RGA driver.

Signed-off-by: Jacob Chen <jacob-chen@iotwrt.com>
[hans.verkuil@cisco.com: added commit message]
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>