ANDROID: sched: Introduce sysctl_sched_cstate_aware

Introduce a new sysctl for this option, 'sched_cstate_aware'.
When this is enabled, the scheduler can make use of the idle state
indexes in order to break the tie between potential CPU candidates.

This patch is based on 7f6fb825d6bc ("ANDROID: sched: EAS: take cstate
into account when selecting idle core") from android-4.14. All the
credits goes to the authors.

Change-Id: Ia076cf32faff91e90905291fa6f7924dc3dd6458
Signed-off-by: Quentin Perret <quentin.perret@arm.com>