Define the generic R8A77970 part of the DU device node.

Based on the original (and large) patch by Daisuke Matsushita
<daisuke.matsushita.ns@hitachi.com>.
Signed-off-by: NVladimir Barinov <vladimir.barinov@cogentembedded.com>
Signed-off-by: NSergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: NLaurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: NSimon Horman <horms+renesas@verge.net.au>