core_ctl: Synchronize access to cluster cpu list

Cluster cpu list traversal is not properly protected against removal of
element by a separate thread. Add proper locking to ensure an element
cannot be removed while accessing the list.

In addition ensure we don't end up in a livelock never exiting the loop
due to hotplug continuously moving elements to the end of the list.

Change-Id: Ie98fe48c2f4fdd0244573229b77ee9823df9e214
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>