media: rcar_drif: convert to SPDX identifiers

As original license mentioned, it is GPL-2.0+ in SPDX.
Then, MODULE_LICENSE() should be "GPL" instead of "GPL v2".
See ${LINUX}/include/linux/module.h

	"GPL"		[GNU Public License v2 or later]
	"GPL v2"	[GNU Public License v2]

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Ramesh Shanmugasundaram <ramesh.shanmugasundaram@bp.renesas.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>