sched: scheduler-driven cpu frequency selection

Scheduler-driven cpu frequency selection is desirable as part of the
on-going effort to make the scheduler better aware of energy
consumption.  No piece of the Linux kernel has a better view of the
factors that affect a cpu frequency selection policy than the
scheduler[0], and this patch is an attempt to converge on an initial
solution.

This patch implements a simple shim layer between the Linux scheduler
and the cpufreq subsystem. This interface accepts a capacity request
from the Completely Fair Scheduler and honors the max request from all
cpus in the same frequency domain.

The policy magic comes from choosing the cpu capacity request from cfs
and is not contained in this cpufreq governor. This code is
intentionally dumb.

Note that this "governor" is event-driven. There is no polling loop to
check cpu idle time nor any other method which is unsynchronized with
the scheduler.

Thanks to Juri Lelli <juri.lelli@arm.com> for contributing design ideas,
code and test results.

[0] http://article.gmane.org/gmane.linux.kernel/1499836



Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Signed-off-by: Juri Lelli <juri.lelli@arm.com>