iommu: Remove config dependency

CONFIG_IOMMU_TESTS and CONFIG_IOMMU_DEBUG_TRACKING should be independent.
Fix a compilation error which results when just
CONFIG_IOMMU_DEBUG_TRACKING is enabled.

Change-Id: I943ce04ad69de63e4881c50ed5d554403c0f0372
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>