scripts/kernel-doc.py: move file lists to the parser function

scripts/kernel-doc.py: move file lists to the parser function

Instead of setting file lists at __init__ time, move it to
the actual parsing function. This allows adding more files
to be parsed in real time, by calling parse function multiple
times.

With the new way, the export_files logic was rewritten to
avoid parsing twice EXPORT_SYMBOL for partial matches.

Please notice that, with this logic, it can still read the
same file twice when export_file is used.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/ab10bc94050406ce6536d4944b5d718ecd70812f.1744106242.git.mchehab+huawei@kernel.org