iommu/arm-smmu: only allow one ATOS operation at a time

Some hardware prohibits multiple ATOS commands from being issued
concurrently.  Add a lock to avoid concurrent ATOS commands from being
issued.

Change-Id: Ie728464f8f03b3d60807a75b9e7c2b50ab62fa43
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>