PM / devfreq: Fix race condition between suspend/resume and governor_store

There is a race condition when the event governor_store is being executed
from sysfs and the device issues a suspend. The devfreq data structures
would become stale when the suspend tries to access them in the middle
of the governor_store operation. Fix this issue by taking a lock around
suspend and resume operations so that these operations are not concurrent
with the other events from sysfs.

Also rename the sysfs_lock as event_lock since the same lock is used
for non sysfs operations like suspend and resume as well.

Change-Id: Ifa0e93915a920cec3e0429966328a1128d61098b
Signed-off-by: Rama Aparna Mallavarapu <aparnam@codeaurora.org>