libfdt: Update to latest pylibfdt implementation

The enhanced pylibfdt support in U-Boot needed for binman was a
placeholder while upstreaming of this work continued. This is now
complete, so bring in the changes and update the tools as needed.

There are quite a few changes since we decided to split the
implementation into three fdt classes instead of two.

The Fdt.del_node() method was unfortunately missed in this process and
will be dealt with later. It exists in U-Boot but not upstream.

Further syncing of libfdt probably needs to wait until we assess the
code-size impact of all the new checking code on SPL and possibly provide
a way to disable it.

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