rtc: sun4v: switch to SPDX identifier

Use SPDX-License-Identifier to be clearer on the license. Choose the v2
only as this is the default Linux license.
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-sun4v.c

+// SPDX-License-Identifier: GPL-2.0
 /* rtc-sun4v.c: Hypervisor based RTC for SUN4V systems.
  *
  * Author: David S. Miller
- * License: GPL
  *
  * Copyright (C) 2008 David S. Miller <davem@davemloft.net>
  */