drm: rcar-du: Add r8a779a0 device support

Extend the rcar_du_device_info structure and rcar_du_output enum to
support DSI outputs and utilise these additions to provide support for
the R8A779A0 V3U platform.

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