drm: rcar-du: Add R-Car DSI driver

The driver supports the MIPI DSI/CSI-2 TX encoder found in the R-Car V3U
SoC. It currently supports DSI mode only.

Signed-off-by: LUU HOAI <hoai.luu.ub@renesas.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Tested-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>