arm64: switch copyright boilerplace to SPDX in dma-mapping.c

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>