sched, cpufreq: Introduce current cpu compute capacity into scheduler

The scheduler is currently unaware of frequency changes and the current
compute capacity offered by the cpus. This patch is not the solution.
It is a hack to give us something to experiment with for now.

A proper solution could be based on the frequency invariant load
tracking proposed in the past: https://lkml.org/lkml/2013/4/16/289



This patch should _not_ be considered safe.

Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>