drm: rcar-du: Add dependency on OF

The driver requires OF support, add a dependency in Kconfig and remove
the platform_device_id table that isn't used anymore.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>