iommu/iommu-debug: Add 3 more signifigant digits

Print time with ns precision.

Ex:
    size           iommu_map      iommu_unmap
      4K           43.021 us        99.479 us

Change-Id: Ib7f3db2be533c77adf0a20de557615a0e2433dd9
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>