PM / devfreq: governor_cpufreq: Update devfreq when cpu offline

Current cpufreq driver will not send CPUFREQ_REMOVE_POLICY
notification when all related cpu offlined, so use
paired notification of CPUFREQ_START and CPUFREQ_STOP
instead to handle the policy start and stop events.

Change-Id: I7cd9b666a282579c7fa10e628da95e77da1b56c5
Signed-off-by: Maria Yu <aiquny@codeaurora.org>