FROMLIST: sched/topology: Disable EAS on inappropriate platforms

Energy Aware Scheduling (EAS) in its current form is most relevant on
platforms with asymmetric CPU topologies (e.g. Arm big.LITTLE) since
this is where there is a lot of potential for saving energy through
scheduling. This is particularly true since the Energy Model only
includes the active power costs of CPUs, hence not providing enough data
to compare packing-vs-spreading strategies.

As such, disable EAS on root domains where the SD_ASYM_CPUCAPACITY flag
is not set. While at it, disable EAS on systems where the complexity of
the Energy Model is too high since that could lead to unacceptable
scheduling overhead.

All in all, EAS can be used on a root domain if and only if:
  1. an Energy Model is available;
  2. the root domain has an asymmetric CPU capacity topology;
  3. the complexity of the root domain's EM is low enough to keep
     scheduling overheads low.

Change-Id: Ia557fbb226be44ed40d7d22661773326276bf9c8
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Quentin Perret <quentin.perret@arm.com>
Message-Id: <20181016101513.26919-8-quentin.perret@arm.com>
Signed-off-by: Quentin Perret <quentin.perret@arm.com>