Treewide: Fix incorrect SPDX license strings

These strings didn't match the license names exactly, so update them
to match.

Change-Id: Ib946eb15ca5fa64cbd6b657350b989b4a4c1b7b7
Signed-off-by: Martin Roth <gaumless@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/80583
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>

util/scripts/parse-maintainers.pl

@@ -1,5 +1,5 @@
 #!/usr/bin/env perl
-# SPDX-License-Identifier: GPL-2.0
+# SPDX-License-Identifier: GPL-2.0-only
 
 use strict;
 use warnings;