iommu/iommu-debug: Improve statistics collection

Add a nr_iters file which specifies the number of runs to average
performance over. Alternatively, setting this to 1 allows the user
to collect raw data.

Change-Id: Ifab7215027b597fb2650ab2f0c5065bebe0b8c5d
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>