iommu/iommu-debug: Set DOMAIN_ATTR_ATOMIC during profiling

There's some non-deterministic latency incurred by enabling clocks and
regulators.  Clocks and regulators are left on for atomic domains, which
makes them a good choice for profiling the page table management code.
Use an atomic domain for map/unmap profiling to make things more
deterministic.

Change-Id: Ib9fa48699c9d6009dd3f3b322002e2fb0f2ff99c
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>