ethernet: renesas: convert to SPDX identifiers

This patch updates license to use SPDX-License-Identifier
instead of verbose license text.
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/renesas/Kconfig

+# SPDX-License-Identifier: GPL-2.0
 #
 # Renesas device configuration
 #

drivers/net/ethernet/renesas/Makefile

+# SPDX-License-Identifier: GPL-2.0
 #
 # Makefile for the Renesas device drivers.
 #

drivers/net/ethernet/renesas/ravb_ptp.c

+// SPDX-License-Identifier: GPL-2.0+
 /* PTP 1588 clock using the Renesas Ethernet AVB
  *
  * Copyright (C) 2013-2015 Renesas Electronics Corporation
  * Copyright (C) 2015 Renesas Solutions Corp.
  * Copyright (C) 2015-2016 Cogent Embedded, Inc. <source@cogentembedded.com>
- *
- *  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.
  */
 
 #include "ravb.h"