drm: rcar-du: Add DPLL support

The implementation hardcodes a workaround for the H3 ES1.x SoC
regardless of the SoC revision, as the workaround can be safely applied
on all devices in the Gen3 family without any side effect.

Signed-off-by: Koji Matsuoka <koji.matsuoka.xm@renesas.com>
Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>