sched: Introduce sched_freq_aggregate_threshold tunable

Do the aggregation for frequency only when the total group busy time
is above sched_freq_aggregate_threshold. This filtering is especially
needed for the cases where groups are created by including all threads
of an application process. This knob can be tuned to apply aggregation
only for the heavy workload applications.

When this knob is enabled and load is aggregated, the load is not
clipped to 100% @ current frequency to ramp up the frequency faster.

Change-Id: Icfd91c85938def101a989af3597d3dcaa8026d16
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>