sched: Add snapshot of Window Assisted Load Tracking (WALT)

This snapshot is taken from msm-4.14 as of
commit 871eac76 ("sched: Improve the scheduler").

Change-Id: Ib4e0b39526d3009cedebb626ece5a767d8247846
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>