MAINTAINERS: rcar-du: Add co-maintainer

Add myself as a co-maintainer for the Renesas DRM drivers.

Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Acked-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>