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: I85b232b5c4dbf65bcb1fede97e1894e6ef9b6a94
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>