sched: fix CPU frequency estimation while idle

CPU cycle counter won't increase when CPU or cluster is idle depending
on hardware.  Thus using cycle counter in that period of time can
result in incorrect CPU frequency estimation.  Use previously calculated
CPU frequency when CPU was idle.

Change-Id: I732b50c974a73c08038995900e008b4e16e9437b
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>