dt: add 'const' for of_property_read_string parameter **out_string

The existing dt codes usually call of_get_property to get a string
property and save it as a 'const char *'.  The patch adds'const' for
of_property_read_string parameter **out_string to make the converting
of existing code a little easier.
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

drivers/of/base.c

@@ -639,7 +639,7 @@ EXPORT_SYMBOL_GPL(of_property_read_u32);
  * The out_string pointer is modified only if a valid string can be decoded.
  */
 int of_property_read_string(struct device_node *np, char *propname,
-				char **out_string)
+				const char **out_string)
 {
 	struct property *prop = of_find_property(np, propname, NULL);
 	if (!prop)

include/linux/of.h

@@ -198,7 +198,7 @@ extern struct property *of_find_property(const struct device_node *np,
 extern int of_property_read_u32(struct device_node *np, char *propname,
 					u32 *out_value);
 extern int of_property_read_string(struct device_node *np, char *propname,
-					char **out_string);
+					const char **out_string);
 extern int of_device_is_compatible(const struct device_node *device,
 				   const char *);
 extern int of_device_is_available(const struct device_node *device);