FROMLIST: sched: Introduce sched_energy_present static key

In order to make sure Energy Aware Scheduling (EAS) will not impact
systems where no Energy Model is available, introduce a static key
guarding the access to EAS code. Since EAS is enabled on a
per-root-domain basis, the static key is enabled when at least one root
domain meets all conditions for EAS.

Change-Id: Ifa3e490e023d3f57b2f1b1272d5ea58d6ae726ab
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Quentin Perret <quentin.perret@arm.com>
Message-Id: <20181016101513.26919-10-quentin.perret@arm.com>
Signed-off-by: Quentin Perret <quentin.perret@arm.com>