drm: rcar-du: Add R8A7795 device support

Document the R8A7795-specific DT bindings and support them in the
driver. The HDMI and LVDS outputs are currently not supported.

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