drm: rcar-du: lvds: add R8A77980 support

Add support for the R-Car V3H (R8A77980) SoC to the LVDS encoder driver.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>