coresight: add save restore support for multi cluster cpu ctis

Modify the CoreSight CTI driver to save and restore only those cpus
belonging to the cluster for which L2 SCU rail is powered off. Also
prevent access to CTI registers for those CPUs whose L2 rail is
powered down.

Change-Id: I4a7ba242b6897962e5cc414e94afc832c464e164
Signed-off-by: Aparna Das <adas@codeaurora.org>