PM / OPP: Initialize OPP table from device tree

With a lot of devices booting from device tree nowadays, it requires
that OPP table can be initialized from device tree.  The patch adds
a helper function of_init_opp_table together with a binding doc for
that purpose.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
diff --git a/include/linux/opp.h b/include/linux/opp.h
index 2a4e5fa..214e0ebc 100644
--- a/include/linux/opp.h
+++ b/include/linux/opp.h
@@ -48,6 +48,14 @@
 
 struct srcu_notifier_head *opp_get_notifier(struct device *dev);
 
+#ifdef CONFIG_OF
+int of_init_opp_table(struct device *dev);
+#else
+static inline int of_init_opp_table(struct device *dev)
+{
+	return -EINVAL;
+}
+#endif /* CONFIG_OF */
 #else
 static inline unsigned long opp_get_voltage(struct opp *opp)
 {