treewide: more SPDX header work

Change-Id: Ib78c322730ec6dfa9dcaafa16e5741cd3d351b8d
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41174
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>

gnat.adc

@@ -1,15 +1,5 @@
---
 -- This file is part of the coreboot project.
---
--- This program is free software; you can redistribute it and/or modify
--- it under the terms of the GNU General Public License as published by
--- the Free Software Foundation; version 2 of the License.
---
--- This program is distributed in the hope that it will be useful,
--- but WITHOUT ANY WARRANTY; without even the implied warranty of
--- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
--- GNU General Public License for more details.
---
+-- SPDX-License-Identifier: GPL-2.0-only
 
 pragma Restrictions (No_Access_Subprograms);
 pragma Restrictions (No_Allocators);