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.

Change-Id: I376dffa1e7f3f47d0496cd7e6ef8b5642ab79016
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>