drm: rcar-du: Convert to SPDX identifiers

Kconfig doesn't have license line, thus, it is GPL-2.0 as default.
rcar_du_regs.h, rcar_lvds_regs.h are GPL-2.0,
and all other files are GPL-2.0+ as original license.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>