src/arch/ppc64: Convert to SPDX license header

This also drops individual copyright notices, all mentioned authors in
that part of the tree are already listed in AUTHORS.

Change-Id: I19b1c379b474dd011e2d0f8c8202ff1351c9290d
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39281
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/arch/ppc64/boot.c

@@ -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 */
+/* This file is part of the coreboot project. */
 
 #include <program_loading.h>