iommu/dma: Convert to address-based allocation

In preparation for some IOVA allocation improvements, clean up all the
explicit struct iova usage such that all our mapping, unmapping and
cleanup paths deal exclusively with addresses rather than implementation
details. In the process, a few of the things we're touching get renamed
for the sake of internal consistency.

Reviewed-by: Nate Watterson <nwatters@codeaurora.org>
Tested-by: Nate Watterson <nwatters@codeaurora.org>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>