MIPS: fix SPDX license identifier in remaining arch header files

Add a SPDX license identifier to MIPS header files where it is
still missing.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
diff --git a/arch/mips/include/asm/bitops.h b/arch/mips/include/asm/bitops.h
index c31ff6e..cce6995 100644
--- a/arch/mips/include/asm/bitops.h
+++ b/arch/mips/include/asm/bitops.h
@@ -1,10 +1,8 @@
 /*
- * This file is subject to the terms and conditions of the GNU General Public
- * License.  See the file "COPYING" in the main directory of this archive
- * for more details.
- *
  * Copyright (c) 1994 - 1997, 1999, 2000  Ralf Baechle (ralf@gnu.org)
  * Copyright (c) 2000  Silicon Graphics, Inc.
+ *
+ * SPDX-License-Identifier:	GPL-2.0
  */
 #ifndef _ASM_BITOPS_H
 #define _ASM_BITOPS_H