dm: core: Avoid creating a name property when unflattening

The current implementation creates a 'name' value for every node. This
is not needed for the latest device tree format, which includes a name in
the node header.

Adjust the code to point the name at the node header instead.

Also simplify ofnode_get_name(), now that we can rely on it to set the
name correctly. Update the comment to make it clear what name the root
node has.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/dm/ofnode.h

@@ -466,7 +466,7 @@ ofnode ofnode_get_parent(ofnode node);
  * ofnode_get_name() - get the name of a node
  *
  * @node: valid node to look up
- * Return: name of node
+ * Return: name of node (for the root node this is "")
  */
 const char *ofnode_get_name(ofnode node);