dmaengine: axi-dmac: update license header

The change replaces the old license information in the comment header with
the new SPDX license specifier.
As well as bumping the year range from 2013-2015 to 2013-2019.

The latter also reflects recent changes that were added to the driver.
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/dma-axi-dmac.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * Driver for the Analog Devices AXI-DMAC core
  *
- * Copyright 2013-2015 Analog Devices Inc.
+ * Copyright 2013-2019 Analog Devices Inc.
  *  Author: Lars-Peter Clausen <lars@metafoo.de>
- *
- * Licensed under the GPL-2.
  */
 
 #include <linux/clk.h>