platform/chrome: cros_ec_vbc: switch to SPDX identifier

Adopt the SPDX license identifier headers to ease license compliance
management.
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>

drivers/platform/chrome/cros_ec_vbc.c

-/*
- * cros_ec_vbc - Expose the vboot context nvram to userspace
- *
- * Copyright (C) 2015 Collabora Ltd.
- *
- * based on vendor driver,
- *
- * Copyright (C) 2012 The Chromium OS Authors
- *
- * 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; either version 2 of the License, or
- * (at your option) any later version.
- *
- * 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+
+// Expose the vboot context nvram to userspace
+//
+// Copyright (C) 2012 Google, Inc.
+// Copyright (C) 2015 Collabora Ltd.
 
 #include <linux/of.h>
 #include <linux/platform_device.h>