of: Add of_property_match_string() to find index into a string list

Add a helper function for finding the index of a string in a string
list property.  This helper is useful for bindings that use a separate
*-name property for attaching names to tuples in another property such
as 'reg' or 'gpios'.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
diff --git a/include/linux/of.h b/include/linux/of.h
index a75a831..5a4a3ad 100644
--- a/include/linux/of.h
+++ b/include/linux/of.h
@@ -217,6 +217,9 @@
 extern int of_property_read_string_index(struct device_node *np,
 					 const char *propname,
 					 int index, const char **output);
+extern int of_property_match_string(struct device_node *np,
+				    const char *propname,
+				    const char *string);
 extern int of_property_count_strings(struct device_node *np,
 				     const char *propname);
 extern int of_device_is_compatible(const struct device_node *device,