PM / devfreq: add locking documentation for recommended_opp

OPP pointers are protected by RCU locks, the pointer validity is
permissible only under the section of rcu_read_lock to rcu_read_unlock

Add documentation to the effect.
Signed-off-by: NNishanth Menon <nm@ti.com>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/devfreq/devfreq.c

@@ -994,6 +994,11 @@ module_exit(devfreq_exit);
  * @freq:	The frequency given to target function
  * @flags:	Flags handed from devfreq framework.
  *
+ * Locking: This function must be called under rcu_read_lock(). opp is a rcu
+ * protected pointer. The reason for the same is that the opp pointer which is
+ * returned will remain valid for use with opp_get_{voltage, freq} only while
+ * under the locked area. The pointer returned must be used prior to unlocking
+ * with rcu_read_unlock() to maintain the integrity of the pointer.
  */
 struct opp *devfreq_recommended_opp(struct device *dev, unsigned long *freq,
 				    u32 flags)