ANDROID: sched, cpuidle: Track cpuidle state index in the scheduler

The idle-state of each cpu is currently pointed to by rq->idle_state but
there isn't any information in the struct cpuidle_state that can used to
look up the idle-state energy model data stored in struct
sched_group_energy. For this purpose is necessary to store the idle
state index as well. Ideally, the idle-state data should be unified.

cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
Change-Id: Ib3d1178512735b0e314881f73fb8ccff5a69319f
Signed-off-by: Chris Redpath <chris.redpath@arm.com>
(cherry picked from commit a732c97420e109956c20f34c70b91e6d06f5df31)
[ Fixed trivial cherry-pick conflict in kernel/sched/sched.h ]
Signed-off-by: Quentin Perret <quentin.perret@arm.com>