sched/core_ctl: introduce nr_prev_assist_thresh tunable

Higher capacity cluster need to help lower capacity cluster
if tasks running on lower capacity cluster are more than certain
threshlod value eventhough tasks doesn't show demand for higher
capacity cluster to achieve better performance.

Change-Id: Icafe448dbd69f8748888b0254702b33ebe80054d
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>