sched: Packing support until a frequency threshold

Add another dimension for task packing based on frequency. This patch
adds a per-cpu tunable, rq->mostly_idle_freq, which when set will
result in tasks being packed on a single cpu in cluster as long as
cluster frequency is less than set threshold.

Change-Id: I8c65376801efd158c8145073a10a1a555004f1da
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>