PM / OPP: add dev_pm_opp_get_suspend_opp() helper

Add dev_pm_opp_get_suspend_opp() helper to obtain suspend opp.

Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
(cherry picked from commit 4eafbd15b6c84cd3f6c76022c8a6c27f7cc076e1)
Signed-off-by: Alex Shi <alex.shi@linaro.org>