pylibfdt: correct license information (further)

pylibfdt: correct license information (further)

Since commit 51ec8db23280 ("pylibfdt: correct license information"), the
License classifiers are gone so I assume setuptools now extract the
license from the license argument to setuptools.setup() function.

It's always been incorrect as far as I could tell, so let's fix this
with the appropriate info from the SPDX License identifier at the top of
the file. It was missing GPL-2.0-or-later and we disambiguate by using
BSD-2-Clause instead of simply BSD.

Fixes: 6b08fb5cc44f ("fdt: Move to setuptools")
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>