sched: restore discarded ifdef CONFIG_SCHED_WALT code

Code closed in ifdef CONFIG_SCHED_WALT blocks is not used in
msm-4.4 builds, hence in order to be as much as closer to
upstream and subsequently to have less merge conflicts in the
future, let's restore this code.

Restore below CONFIG_SCHED_WALT changes in file [1]:

  3c5c4e97 trace/sched: add rq utilization signal for WALT
  3a29814d sched: fix wrong truncation of walt_avg
  efb86bd0 sched: Introduce Window Assisted Load Tracking (WALT)

[1] include/trace/events/sched.h

The above changes were discarded during android-4.4 merging
into msm-4.4 starting from change 1758716.

Change-Id: I1dfe59689ee5f1207e0951a8362a497a8c67c6a7
Signed-off-by: Blagovest Kolenichev <bkolenichev@codeaurora.org>