dm: core: Avoid creating a name property when unflattening

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>