scripts: add an script to parse the ABI files

Add a script to parse the Documentation/ABI files and produce
an output with all entries inside an ABI (sub)directory.

Right now, it outputs its contents on ReST format. It shouldn't
be hard to make it produce other kind of outputs, since the ABI
file parser is implemented in separate than the output generator.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>