dm: core: Drop the const from ofnode

Now that we support writing to ofnodes, the const is not accurate. Drop
it to avoid undesirable casting.

Also drop the ofnode_to_npw() which is now the same as ofnode_to_np().

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/dm/ofnode_decl.h b/include/dm/ofnode_decl.h
index 266253d..8d0d788 100644
--- a/include/dm/ofnode_decl.h
+++ b/include/dm/ofnode_decl.h
@@ -39,7 +39,7 @@
  *	is not a really a pointer to a node: it is an offset value. See above.
  */
 typedef union ofnode_union {
-	const struct device_node *np;
+	struct device_node *np;
 	long of_offset;
 } ofnode;