iommu: Tidy up io-pgtable dependencies

Some io-pgtable implementations, and thus their users too, carry a
slightly odd dependency to get around the GENERIC_ATOMIC64 version of
cmpxchg64() often failing to compile. Since this is a functional
dependency, it's a bit misleading and untidy to tie it explicitly to
COMPILE_TEST while assuming that it's also implied by the other
platform/architecture options. Make things clearer by separating these
functional dependencies into distinct statements from those controlling
visibility, and since they do look a bit non-obvious to the uninitiated,
also commenting them for good measure.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/51d8c78e2ecc6696ac5907526580209ea6da167f.1673553587.git.robin.murphy@arm.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>