PM / OPP: export dev_pm_opp_get_notifier

Allows user drivers such as devfreq to be modules.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
(cherry picked from commit 4679ec3727a0eb4d57e23dffa8e19ce911362c9f)
Signed-off-by: Alex Shi <alex.shi@linaro.org>