util/lint & LICENSES: Add PDDC as a "license" for coreboot

util/lint & LICENSES: Add PDDC as a "license" for coreboot

The Creative Commons Public Domain Dedication and Certification is not
a license in the common sense in that it's stating that the associated
file is already in the public domain (having no copyright), and is not
actually putting it in the public domain like the CC-C0 license does.

The use for this in coreboot is for unlicensable files - either blank
files or files with no creative content.  This allows these files to
have the SPDX identifier to identify them as having no known copyright
for open source license compliance.

If CC-PDM-1.0 is ever included in the list of SPDX licenses, that would
probably fit better, but because the public domain mark isn't actually
a license, and because "public domain" isn't well defined, CC-PDM was
rejected as a SPDX identifier.

For further information:
https://web.archive.org/web/20201018194411/https://github.com/spdx/license-list-XML/issues/988

Change-Id: Ibb300ecd066cde2a016195c2beca76a460c588e3
Signed-off-by: Martin Roth <martin.roth@amd.corp-partner.google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/66496
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>