tools/power turbostat: allow sub-sec intervals

BugLink: http://bugs.launchpad.net/bugs/1591802



turbostat -i interval_sec

will sample and display statistics every interval_sec.
interval_sec used to be a whole number of seconds,
but now we accept a decimal, as small as 0.001 sec (1 ms).

Signed-off-by: Len Brown <len.brown@intel.com>
(cherry picked from commit 2a0609c0)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Brad Figg <brad.figg@canonical.com>
Acked-by: Kamal Mostafa <kamal@canonical.com>