PM / devfreq: governor_cache_hwmon: Fix race in monitor start/stop

Some cache_hwmon devices can have interrupts firing at any time. The
interrupt handler would stop devfreq monitor, update its vote and
restart the monitor again. This introduces a race if
devfreq_supend/resume() or devfreq_interval_update() is called at
the same time. Since devfreq_monitor_start() re-initializes the work,
it could cause corruption while the work is being used elsewhere.

Protect governor monitor start/stops with a new lock.

Change-Id: I143aaaea86494b4c617df46e2c521a19b43861d5
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>
Signed-off-by: Rama Aparna Mallavarapu <aparnam@codeaurora.org>