drm: rcar-du: lvds: Add reset control

Reset LVDS using the reset control as CPG reset/release is required in
the hardware manual sequence.

Based on a BSP patch from Koji Matsuoka <koji.matsuoka.xm@renesas.com>.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>