drm/rcar-du: Add support for multiple groups

The R8A7790 DU has 3 CRTCs, split in two groups. Support them.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
diff --git a/drivers/gpu/drm/rcar-du/rcar_du_group.h b/drivers/gpu/drm/rcar-du/rcar_du_group.h
index 180c739..4487e83 100644
--- a/drivers/gpu/drm/rcar-du/rcar_du_group.h
+++ b/drivers/gpu/drm/rcar-du/rcar_du_group.h
@@ -38,6 +38,9 @@
 	struct rcar_du_planes planes;
 };
 
+u32 rcar_du_group_read(struct rcar_du_group *rgrp, u32 reg);
+void rcar_du_group_write(struct rcar_du_group *rgrp, u32 reg, u32 data);
+
 int rcar_du_group_get(struct rcar_du_group *rgrp);
 void rcar_du_group_put(struct rcar_du_group *rgrp);
 void rcar_du_group_start_stop(struct rcar_du_group *rgrp, bool start);