coresight: fix the mutex deadlock issue in cpu hotplug path

The CPU hotplug notification path is called up by holding the
cpu_add_remove_lock and when the same is held by the
unregister_hotcpu_notifier() in the notifier path results in
deadlock.So, release and acquire the cpu_add_remove_lock before and
after unregistration of cpu notifiers.

Change-Id: I46d6b2ae98dc1eae7c55f8ac62325d25b48d463f
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>