FROMLIST: sched/fair: Introduce an energy estimation helper function

In preparation for the definition of an energy-aware wakeup path,
introduce a helper function to estimate the consequence on system energy
when a specific task wakes-up on a specific CPU. compute_energy()
estimates the capacity state to be reached by all performance domains
and estimates the consumption of each online CPU according to its Energy
Model and its percentage of busy time.

Change-Id: Ia291deb16ec9a75f3c0252abbb5d864e3300562d
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Quentin Perret <quentin.perret@arm.com>
Message-Id: <20181016101513.26919-14-quentin.perret@arm.com>
Signed-off-by: Quentin Perret <quentin.perret@arm.com>