sched: Make scheduler aware of cpu frequency state

Capacity of a cpu (how much performance it can deliver) is partly
determined by its frequency (P) state, both current frequency as well
as max frequency it can reach.  Knowing frequency state of cpus will
help scheduler optimize various functions such as tracking every
task's cpu demand and placing tasks on various cpus.

This patch has scheduler registering for cpufreq notifications to
become aware of cpu's frequency state. Subsequent patches will make
use of derived information for various purposes, such as task's scaled
load (cpu demand) accounting and task placement.

Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>

Conflicts:
	kernel/sched/core.c
sched: window-based load stats for tasks

Provide a metric per task that specifies how cpu bound a task is. Task
execution is monitored over several time windows and the fraction of
the window for which task was found to be executing or wanting to run
is recorded as task's demand. Windows over which task was sleeping are
ignored. We track last 5 recent windows for every task and the maximum
demand seen in any of the previous 5 windows (where task had some
activity) drives freq demand for every task.

A per-cpu metric (rq->cumulative_runnable_avg) is also provided which
is an aggregation of cpu demand of all tasks currently enqueued on it.
rq->cumulative_runnable_avg will be useful to know if cpu frequency
will need to be changed to match task demand.

Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>

Conflicts:
	include/linux/sched/sysctl.h
sysctl.h: add header
sysctl: fix merge derp