tools/power turbostat: Fix and document --header_iterations

tools/power turbostat: Fix and document --header_iterations

The "header_iterations" option is commonly used to de-clutter
the screen of redundant header label rows in an interactive session:
Eg. every 10 rows:

$ sudo turbostat --header_iterations 10 -S -q -i 1

But --header_iterations was missing from turbostat.8

Also turbostat help advertised the "-N" short option
that did not actually work:

$ turbostat --help
  -N, --header_iterations num
		print header every num iterations

Repair "-N"
Document "--header_iterations" on turbostat.8

Signed-off-by: Len Brown <len.brown@intel.com>