arm: debug: replace license blocks with SPDX

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/arch/arm/lib/debug.S b/arch/arm/lib/debug.S
index 14f7c3b..ddee7c0 100644
--- a/arch/arm/lib/debug.S
+++ b/arch/arm/lib/debug.S
@@ -3,9 +3,7 @@
  *
  *  Copyright (C) 1994-1999 Russell King
  *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License version 2 as
- * published by the Free Software Foundation.
+ * SPDX-License-Identifier:	GPL-2.0+
  *
  *  32-bit debugging code
  */