sched: Provide a facility to restrict RT tasks to lower power cluster

The current CPU selection algorithm for RT tasks looks for the
least loaded CPU in all clusters. Stop the search at the lowest
possible power cluster based on "sched_restrict_cluster_spill"
sysctl tunable.

Change-Id: I34fdaefea56e0d1b7e7178d800f1bb86aa0ec01c
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>