drm: rcar-du: lvds: Add R8A77970 support

Add support for the R-Car V3M (R8A77970) SoC to the LVDS encoder driver.
Note that there are some differences with the other R-Car gen3 SoCs, e.g.
LVDPLLCR has the same layout as in the R-Car gen2 SoCs.

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>