sched: Compute cpu capacity available at current frequency

capacity_orig_of() returns the max available compute capacity of a cpu.
For scale-invariant utilization tracking and energy-aware scheduling
decisions it is useful to know the compute capacity available at the
current OPP of a cpu.

Change-Id: I95b37102016184b4252b8f370527cf332bcb5179
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
(am from https://patchwork.kernel.org/patch/7804931/

)
Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>