drm: rcar-du: Add RZ/G2L DSI driver

This driver supports the MIPI DSI encoder found in the RZ/G2L
SoC. It currently supports DSI video mode only.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>