fdtput: Add -c option to create nodes

This option allows the creation of new nodes in a dtb file. The syntax
is:

   fdtput -c <dtb_file> <node_path>

The node_path contains the path of the node to be created. All path
components up to the final one must exist already. The final one must
not exist already.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: David Gibson <david@gibson.dropbear.id.au>