sched: use ktime instead of sched_clock for load tracking

At present, HMP scheduler uses sched_clock to setup window boundary to
be aligned with timer interrupt to ensure timer interrupt fires after
window rollover.  However this alignment won't last long since the timer
interrupt rearms next timer based on time measured by ktime which isn't
coupled with sched_clock.

Convert sched_clock to ktime to avoid wallclock discrepancy between
scheduler and timer so that we can ensure scheduler's window boundary is
always aligned with timer.

CRs-fixed: 933330
Change-Id: I4108819a4382f725b3ce6075eb46aab0cf670b7e
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>