kernel: add SPDX-License-Identifier lines

Add SPDX-License-Identifier lines to some old kernel
files.

Signed-off-by: Tim Bird <tim.bird@sony.com>
Acked-by: Karim Yaghmour <karim.yaghmour@opersys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

kernel/cpu.c

@@ -1,7 +1,6 @@
+// SPDX-License-Identifier: GPL-2.0
 /* CPU control.
  * (C) 2001, 2002, 2003, 2004 Rusty Russell
- *
- * This code is licenced under the GPL.
  */
 #include <linux/sched/mm.h>
 #include <linux/proc_fs.h>