of: add 'const' for of_property_*_string*() parameter '*np'

The of_property_{read,count,match}_string* family of functions never
modify the struct device_node pointer that is passed in, so there is no
reason for it to be non-const. Equivalent functions for all other types
already take a 'const struct device_node *np'.

Signed-off-by: David Rivshin <drivshin@allworx.com>
Signed-off-by: Rob Herring <robh@kernel.org>
diff --git a/drivers/of/base.c b/drivers/of/base.c
index 017dd94..b299de2 100644
--- a/drivers/of/base.c
+++ b/drivers/of/base.c
@@ -1341,10 +1341,10 @@
  *
  * The out_string pointer is modified only if a valid string can be decoded.
  */
-int of_property_read_string(struct device_node *np, const char *propname,
+int of_property_read_string(const struct device_node *np, const char *propname,
 				const char **out_string)
 {
-	struct property *prop = of_find_property(np, propname, NULL);
+	const struct property *prop = of_find_property(np, propname, NULL);
 	if (!prop)
 		return -EINVAL;
 	if (!prop->value)
@@ -1365,10 +1365,10 @@
  * This function searches a string list property and returns the index
  * of a specific string value.
  */
-int of_property_match_string(struct device_node *np, const char *propname,
+int of_property_match_string(const struct device_node *np, const char *propname,
 			     const char *string)
 {
-	struct property *prop = of_find_property(np, propname, NULL);
+	const struct property *prop = of_find_property(np, propname, NULL);
 	size_t l;
 	int i;
 	const char *p, *end;
@@ -1404,10 +1404,11 @@
  * Don't call this function directly. It is a utility helper for the
  * of_property_read_string*() family of functions.
  */
-int of_property_read_string_helper(struct device_node *np, const char *propname,
-				   const char **out_strs, size_t sz, int skip)
+int of_property_read_string_helper(const struct device_node *np,
+				   const char *propname, const char **out_strs,
+				   size_t sz, int skip)
 {
-	struct property *prop = of_find_property(np, propname, NULL);
+	const struct property *prop = of_find_property(np, propname, NULL);
 	int l = 0, i = 0;
 	const char *p, *end;