libfdt: Bring in proposed pylibfdt changes

libfdt: Bring in proposed pylibfdt changes

This provides various patches sent to the devicetree-compiler mailing list
to enhance the Python bindings. A final version of this patch may be
created once upstreaming is complete, but if it takes too long, this can
act as a placeholder.

New pylibfdt features:
- Support for most remaining, relevant libfdt functions
- Support for sequential-write functions

Changes are applied to existing U-Boot tools as needed.

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