sched_avg: add run queue averaging

Add code to calculate the run queue depth of a cpu and iowait
depth of the cpu.

The scheduler calls in to sched_update_nr_prod whenever there
is a runqueue change. This function maintains the runqueue average
and the iowait of that cpu in that time interval.

Whoever wants to know the runqueue average is expected to call
sched_get_nr_running_avg periodically to get the accumulated
runqueue and iowait averages for all the cpus.

Change-Id: Id8cb2ecf0ed479f090a83ccb72dd59c53fa73e0c
Signed-off-by: Jeff Ohlstein <johlstei@codeaurora.org>