iommu/dma: Document min_align_mask assumption

iommu/dma: Document min_align_mask assumption

iommu-dma does not explicitly reference min_align_mask since we already
assume that will be less than or equal to any typical IOVA granule.
We wouldn't realistically expect to see the case where it is larger, and
that would be non-trivial to support, however for the sake of reasoning
(particularly around the interaction with SWIOTLB), let's clearly
enforce the assumption.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Michael Kelley <mhklinux@outlook.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/dbb4d2d8e5d1691ac9a6c67e9758904e6c447ba5.1709553942.git.robin.murphy@arm.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>