sched: add hook for platform-specific CPU power information

To enable power-aware scheduling, provide a hook/infrastructure
for platforms to communicate CPU power requirements for each
supported CPU frequency. This information is then used to estimate
the cost of running a task on a given CPU.

Currently, an assumption is made that the task will be running
by itself on the CPU. Given the current policy tries to spread
tasks as much as possible this assumption should not be too
far off.

Change-Id: I19f1fa760a0d43222d2880f8aec0508c468b39bb
Signed-off-by: Steve Muckle <smuckle@codeaurora.org>
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>