tools/power turbostat: report CoreThr per measurement interval

The CoreThr column displays total thermal throttling events
since boot time.

Change it to report events during the measurement interval.

This is more useful for showing a user the current conditions.
Total events since boot time are still available to the user via
/sys/devices/system/cpu/cpu*/thermal_throttle/*

Document CoreThr on turbostat.8

Fixes: eae97e05 ("turbostat: Support thermal throttle count print")
Reported-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>
Cc: Chen Yu <yu.c.chen@intel.com>