PM / OPP: Implement of_dev_pm_opp_find_required_opp()

A device's DT node or its OPP nodes can contain a phandle to other
device's OPP node, in the "required-opps" property.

This patch implements a routine to find that required OPP from the node
that contains the "required-opps" property.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
diff --git a/drivers/opp/opp.h b/drivers/opp/opp.h
index 381a4fb..f9eccf9 100644
--- a/drivers/opp/opp.h
+++ b/drivers/opp/opp.h
@@ -187,6 +187,7 @@ struct opp_table {
 };
 
 /* Routines internal to opp core */
+void dev_pm_opp_get(struct dev_pm_opp *opp);
 void _get_opp_table_kref(struct opp_table *opp_table);
 int _get_opp_count(struct opp_table *opp_table);
 struct opp_table *_find_opp_table(struct device *dev);