tools/power turbostat: if --num_iterations, print for specific number of iterations

There's a use case during test to only print specific round of iterations
if --num_iterations is specified, for example, with this patch applied:

turbostat -i 5 -n 4
will capture 4 samples with 5 seconds interval.

[lenb: renamed to --num_iterations from --iterations]

Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Reviewed-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>