pylibfdt: Support device-tree creation/expansion

Add support for fdt_open_into() and fdt_create_empty_tree() from the
Python library. The former is named resize() since it better fits with
what the Python binding actually does.

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