pylibfdt: Reorder functions to match libfdt.h

The ordering of the Python functions loosely matches the corresponding
function in the C header file, but not exactly. As we add more functions
it is easier to track what is missing if they are in the same order.

Move some functions around to achieve this.

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