nb/intel/sandybridge: Use SPDX headers

Note that pei_data.h uses the BSD 3-Clause license:

https://opensource.org/licenses/BSD-3-Clause

Change-Id: I904b343283239af4fdee583bcbea757f59a0cca7
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39777
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
diff --git a/src/northbridge/intel/sandybridge/Makefile.inc b/src/northbridge/intel/sandybridge/Makefile.inc
index 11f1f15..81f3818 100644
--- a/src/northbridge/intel/sandybridge/Makefile.inc
+++ b/src/northbridge/intel/sandybridge/Makefile.inc
@@ -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
+## This file is part of the coreboot project.
 
 ifeq ($(CONFIG_NORTHBRIDGE_INTEL_SANDYBRIDGE),y)