dt-bindings: display: renesas: du: document R8A77980 bindings

Document the R-Car V3H (R8A77980) SoC in the R-Car DU bindings; the DU
hardware has the same topology as in the R-Car V3M (R8A77970).

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>