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>