Perf: Fix hrtimer operation across hotplug

Stop hrtimer associated with CPU clock event when
core goes down. Start the timer when core comes back up.
This is needed to ensure proper software timer operation
across hotplug.

Change-Id: I027bf2718d177fa31be2a7cfe7ef98f93cb4aeda
Signed-off-by: Sheetal Sahasrabudhe <sheetals@codeaurora.org>