SPDX License cleanup for LiMon imported files

A number of network related files were imported from the LiMon
project; these contain a somewhat unclear license statement:

	Copyright 1994 - 2000 Neil Russell.
	(See License)

I analyzed the source code of LiMon v1.4.2 which was used for this
import.  It does not contain any "License" file, but the top level
directory contains a file "COPYING", which turns out to be GPL v2
of June 1991.  So it is legitimate to conclude that the LiMon derived
files are also to be released under GPLv2.  Mark them as such.

Signed-off-by: Wolfgang Denk <wd@denx.de>

include/net.h

@@ -3,7 +3,7 @@
  *
  *	Copyright 1994 - 2000 Neil Russell.
  *	(See License)
- *
+ *	SPDX-License-Identifier:	GPL-2.0
  *
  * History
  *	9/16/00	  bor  adapted to TQM823L/STK8xxL board, RARP/TFTP boot added

net/arp.c

@@ -6,6 +6,7 @@
  *	Copyright 2000 Roland Borde
  *	Copyright 2000 Paolo Scaffardi
  *	Copyright 2000-2002 Wolfgang Denk, wd@denx.de
+ *	SPDX-License-Identifier:	GPL-2.0
  */
 
 #include <common.h>

net/arp.h

@@ -6,6 +6,7 @@
  *	Copyright 2000 Roland Borde
  *	Copyright 2000 Paolo Scaffardi
  *	Copyright 2000-2002 Wolfgang Denk, wd@denx.de
+ *	SPDX-License-Identifier:	GPL-2.0
  */
 
 #ifndef __ARP_H__

net/cdp.c

@@ -6,6 +6,7 @@
  *	Copyright 2000 Roland Borde
  *	Copyright 2000 Paolo Scaffardi
  *	Copyright 2000-2002 Wolfgang Denk, wd@denx.de
+ *	SPDX-License-Identifier:	GPL-2.0
  */
 
 #include <common.h>

net/cdp.h

@@ -6,6 +6,7 @@
  *	Copyright 2000 Roland Borde
  *	Copyright 2000 Paolo Scaffardi
  *	Copyright 2000-2002 Wolfgang Denk, wd@denx.de
+ *	SPDX-License-Identifier:	GPL-2.0
  */
 
 #if defined(CONFIG_CMD_CDP)

net/net.c

@@ -6,6 +6,7 @@
  *	Copyright 2000 Roland Borde
  *	Copyright 2000 Paolo Scaffardi
  *	Copyright 2000-2002 Wolfgang Denk, wd@denx.de
+ *	SPDX-License-Identifier:	GPL-2.0
  */
 
 /*

net/ping.c

@@ -6,6 +6,7 @@
  *	Copyright 2000 Roland Borde
  *	Copyright 2000 Paolo Scaffardi
  *	Copyright 2000-2002 Wolfgang Denk, wd@denx.de
+ *	SPDX-License-Identifier:	GPL-2.0
  */
 
 #include "ping.h"

net/ping.h

@@ -6,6 +6,7 @@
  *	Copyright 2000 Roland Borde
  *	Copyright 2000 Paolo Scaffardi
  *	Copyright 2000-2002 Wolfgang Denk, wd@denx.de
+ *	SPDX-License-Identifier:	GPL-2.0
  */
 
 #ifndef __PING_H__