sched: Introduce an irq_work to report WALT load

Since WALT stats relevant to the cpufreq governor are updated after the
completion of a window, and the governor needs to base its decisions
after having a complete picture of all the CPUs in a policy, introduce
an irq_work callback that's scheduled in the tick. This irq_work callback
will (for all CPUS) update WALT stats to ensure we have the latest for the
most recently completed window, and report those stats to the govneror
as well. This should get us as close as possible to the previous
solution of sched_get_cpus_busy without having to create a new interface.

Change-Id: Ica79396fc5dfc132b62764b3d01f8a507a2b63aa
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>