scripts/spdxcheck: Exclude MAINTAINERS/CREDITS

Listings of maintainers and people who deserve credits are not really
interesting in terms of copyright. The usage of these files outside of the
kernel is pointless and the file format is trivial. No point in chasing
them or slapping a SPDX identifier into them just because.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>