sched: tighten up jiffy to sched_clock mapping

The tick code already tracks exact time a tick is expected
to arrive. This can be used to eliminate slack in the jiffy
to sched_clock mapping that aligns windows between a caller
of sched_set_window and the scheduler itself.

Change-Id: I9d47466658d01e6857d7457405459436d504a2ca
Signed-off-by: Steve Muckle <smuckle@codeaurora.org>