dmaengine: rcar-dmac: convert to SPDX identifiers

This patch is using C++ comment style for SPDX line only,
because driver author want it.
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/sh/rcar-dmac.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * Renesas R-Car Gen2 DMA Controller Driver
  *
  * Copyright (C) 2014 Renesas Electronics Inc.
  *
  * Author: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
- *
- * This is free software; you can redistribute it and/or modify
- * it under the terms of version 2 of the GNU General Public License as
- * published by the Free Software Foundation.
  */
 
 #include <linux/delay.h>