cpufreq: Create DVFS stress test function.

Change-Id: I98897314ff0df9f4fcf22c9d45b80a43268c8bba
Reviewed-on: http://mcrd1-5.corpnet.asus/code-review/master/67932
Reviewed-by: Singhome Lee <singhome_lee@asus.com>
Tested-by: Singhome Lee <singhome_lee@asus.com>