sched: add sched_get_busy, sched_set_window APIs

sched_get_busy() returns the busy time of a cpu during the most
recent completed window.
sched_set_window() will set window size and aligns windows across
all CPUs.

Change-Id: Ic53e27f43fd4600109b7b6db979e1c52c7aca103
Signed-off-by: Steve Muckle <smuckle@codeaurora.org>
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>