rtc: cros-ec: 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>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-cros-ec.c

-/*
- * RTC driver for Chrome OS Embedded Controller
- *
- * Copyright (c) 2017, Google, Inc
- *
- * Author: Stephen Barber <smbarber@chromium.org>
- *
- * This program is free software; you can redistribute it and/or modify it
- * under the terms and conditions of the GNU General Public License,
- * version 2, as published by the Free Software Foundation.
- *
- * This program is distributed in the hope 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
+// RTC driver for ChromeOS Embedded Controller.
+//
+// Copyright (C) 2017 Google, Inc.
+// Author: Stephen Barber <smbarber@chromium.org>
 
 #include <linux/kernel.h>
 #include <linux/mfd/cros_ec.h>