src: Replace remaining GPLv2 long form headers with SPDX header

Change-Id: I4614e9b02a932530fc22912b5cf502d1b699b451
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41188
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/lib/hw-time-timer.adb

@@ -1,16 +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
 
 with Interfaces.C;