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>
diff --git a/net/arp.c b/net/arp.c
index 20c6b2d..21ed31b 100644
--- a/net/arp.c
+++ b/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>
diff --git a/net/arp.h b/net/arp.h
index bfd57e0..3a0a13a 100644
--- a/net/arp.h
+++ b/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__
diff --git a/net/cdp.c b/net/cdp.c
index 3d9559e..2d8fa03 100644
--- a/net/cdp.c
+++ b/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>
diff --git a/net/cdp.h b/net/cdp.h
index ec7315a..95e4ce0 100644
--- a/net/cdp.h
+++ b/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)
diff --git a/net/net.c b/net/net.c
index 722089f..2bea07b 100644
--- a/net/net.c
+++ b/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
  */
 
 /*
diff --git a/net/ping.c b/net/ping.c
index 0710b92..2be56ed 100644
--- a/net/ping.c
+++ b/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"
diff --git a/net/ping.h b/net/ping.h
index 8c71be4..b672b95 100644
--- a/net/ping.h
+++ b/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__