sched: Add provision to control the spreading on SMP

Spreading the tasks across CPUs in different groups/clusters
may hurt power on some SMP systems. Start spreading the tasks
only if all the CPUs in the first group/cluster are utilized
above sched_smp_overlap_capacity threshold. The subsequent
patches add support for deriving this threshold from the
energy-model for optimal power.

Change-Id: I1f77fd9424503166bde268e8e2a0131bbda5d125
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>