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 space.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
(cherry picked from commit 8c2580d6d0454b7980d059af5133bca4763dc8eb)