iommu/io-pgtable-arm-v7s: Support lockless operation

Mirroring the LPAE implementation, rework the v7s code to be robust
against concurrent operations. The same two potential races exist, and
are solved in the same manner, with the fixed 2-level structure making
life ever so slightly simpler.

What complicates matters compared to LPAE, however, is large page
entries, since we can't update a block of 16 PTEs atomically, nor assume
available software bits to do clever things with. As most users are
never likely to do partial unmaps anyway (due to DMA API rules), it
doesn't seem unreasonable for this case to remain behind a serialising
lock; we just pull said lock down into the bowels of the implementation
so it's well out of the way of the normal call paths.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>