sched: prevent race where update CPU cycles

Updating cycle counter should be serialized by holding rq lock.
Add missing rq lock hold when cycle counter is updated by irq entry
point.

Change-Id: I92cf75d047a45ebf15a6ddeeecf8fc3823f96e5d
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>