sched: fix overflow in scaled execution time calculation

Task execution time in nanoseconds and CPU cycle counters are large
enough to cause overflow when we multiply both.  Avoid overflow by
calculating frequency separately.

Change-Id: I076d9ecd27cb1c1f11578f009ebe1a19c1619454
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>