docs: kernel_abi.py: add a script to parse ABI documentation

The ABI documentation is special: it is not plain text files,
but, instead, files with an strict format, as specified by
Documentation/ABI/README.

Add a parser for it.

Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/48abf1a410237e63f85354a8cd7027fdf25657bf.1604042072.git.mchehab+huawei@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>