of: Provide default of_node_to_nid() implementation.

of_node_to_nid() is only relevant in a few architectures.  Don't force
everyone to implement it anyway.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

include/linux/of.h

@@ -146,6 +146,11 @@ static inline unsigned long of_read_ulong(const __be32 *cell, int size)
 
 #define OF_BAD_ADDR	((u64)-1)
 
+#ifndef of_node_to_nid
+static inline int of_node_to_nid(struct device_node *np) { return -1; }
+#define of_node_to_nid of_node_to_nid
+#endif
+
 extern struct device_node *of_find_node_by_name(struct device_node *from,
 	const char *name);
 #define for_each_node_by_name(dn, name) \