sched: Add support to spread tasks

If sysctl_sched_prefer_spread is enabled, then tasks would be freely
migrated to idle cpus within same cluster to reduce runnables.

By default, the feature is disabled.
User can trigger feature with:
   echo 1 > /proc/sys/kernel/sched_prefer_spread
	Aggressively spread tasks with in little cluster.
   echo 2 > /proc/sys/kernel/sched_prefer_spread
	Aggressively spread tasks with in little cluster as well as
	big cluster, but not between big and little.

Change-Id: I0a4d87bd17de3525548765472e6f388a9970f13c
Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>