iommu/dma: Stop getting dma_32bit_pfn wrong

iommu/dma: Stop getting dma_32bit_pfn wrong

iommu_dma_init_domain() was originally written under the misconception
that dma_32bit_pfn represented some sort of size limit for IOVA domains.
Since the truth is almost the exact opposite of that, rework the logic
and comments to reflect its real purpose of optimising lookups when
allocating from a subset of the available 64-bit space.

BugLink: http://bugs.launchpad.net/bugs/1680549
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>
(cherry picked from commit f51d7bb79c1124f7f02e9f472ef935eba13bca8e)
Signed-off-by: Manoj Iyer <manoj.iyer@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>