drm: rcar-du: lvds: Add R-Car Gen3 support

The LVDS encoder differs slightly in Gen3 SoCs in its PLL configuration.
Add support for the Gen3 LVDS PLL parameters and startup procedure.

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