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>