opp: core: Add missing export for dev_pm_opp_adjust_voltage

Export dev_pm_opp_adjust_voltage() as it may be used by modules later
on.

Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
[ Viresh: Rewrote commit log ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>