of: Support const and non-const use for to_of_node()

Turn to_of_node() into a macro in order to support both const and
non-const use. Additionally make the fwnode argument to is_of_node() const
as well.
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Rob Herring <robh@kernel.org>

include/linux/of.h

@@ -148,16 +148,20 @@ extern raw_spinlock_t devtree_lock;
 #ifdef CONFIG_OF
 void of_core_init(void);
 
-static inline bool is_of_node(struct fwnode_handle *fwnode)
+static inline bool is_of_node(const struct fwnode_handle *fwnode)
 {
 	return !IS_ERR_OR_NULL(fwnode) && fwnode->type == FWNODE_OF;
 }
 
-static inline struct device_node *to_of_node(struct fwnode_handle *fwnode)
-{
-	return is_of_node(fwnode) ?
-		container_of(fwnode, struct device_node, fwnode) : NULL;
-}
+#define to_of_node(__fwnode)						\
+	({								\
+		typeof(__fwnode) __to_of_node_fwnode = (__fwnode);	\
+									\
+		is_of_node(__to_of_node_fwnode) ?			\
+			container_of(__to_of_node_fwnode,		\
+				     struct device_node, fwnode) :	\
+			NULL;						\
+	})
 
 #define of_fwnode_handle(node)						\
 	({								\
@@ -539,12 +543,12 @@ static inline void of_core_init(void)
 {
 }
 
-static inline bool is_of_node(struct fwnode_handle *fwnode)
+static inline bool is_of_node(const struct fwnode_handle *fwnode)
 {
 	return false;
 }
 
-static inline struct device_node *to_of_node(struct fwnode_handle *fwnode)
+static inline struct device_node *to_of_node(const struct fwnode_handle *fwnode)
 {
 	return NULL;
 }