Scale CPU usage by CPU frequency

[RAOF]

20% CPU usage at 800MHz is obviously a somewhat different workload to 20% CPU usage at 4300MHz. It would be nice if the CPU utilisation was scaled so that 20% CPU meant the CPU was working at 20% capacity¹, rather than 20% of whatever the current frequency is.

The existence of boost clocks would mean that this could give numbers in excess of 100%, but that's probably ok?

¹: A really accurate measure of capacity is hard to do (and almost certainly processor-specific), but just scaling based on CPU frequency is probably good enough.