thermal-core: Add a lock to tz_notify_trip()

Lock the sensor threshold list during notifications so it
isn't updated until notifications are sent out nor can
something else update thresholds until all notifications
are sent out.

Change-Id: Iafce75fa7fceda3de867044861e4a2e3eef47c95
Signed-off-by: Jeff Bernard <jbernard@codeaurora.org>