dmaengine: mxs-dma: Switch to SPDX identifier

Adopt the SPDX license identifier headers to ease license compliance
management.
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/mxs-dma.c

-/*
- * Copyright 2011 Freescale Semiconductor, Inc. All Rights Reserved.
- *
- * Refer to drivers/dma/imx-sdma.c
- *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License version 2 as
- * published by the Free Software Foundation.
- */
+// SPDX-License-Identifier: GPL-2.0
+//
+// Copyright 2011 Freescale Semiconductor, Inc. All Rights Reserved.
+//
+// Refer to drivers/dma/imx-sdma.c
 
 #include <linux/init.h>
 #include <linux/types.h>