checkpatch: add upstream checkpatch v5.1 dependencies ...

In preperation for pulling in the v5.1 checkpatch, we need to first
add some files it depends on that aren't available in barebox:

- cleanfile, cleanpatch, spelling.txt:
	we copy those over as-is
- spdxcheck.py:
    expects the existence of "other" and "exceptions" directories,
    which we don't need in barebox, thus they are patched out.
- const_structs.checkpatch:
	added file_operations for now. More can follow
- get_maintainer.pl:
	implemented a dummy to keep changes in checkpatch.pl to a minimum

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>