powerpc: Replace GPL 2.0+ README.legal boilerplate with SPDX

Upstream Linux never had a "README.legal" file, but it was present
in early source releases of Linux/m68k.  It contained a simple copyright
notice and a link to a version of the "COPYING" file that predated the
addition of the "only valid GPL version is v2" clause.

Get rid of the references to non-existent files by replacing the
boilerplate with SPDX license identifiers.
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/d91725ff1ed5d4b6ba42474e2ebfeebe711cba23.1695031668.git.geert@linux-m68k.org

arch/powerpc/kernel/ptrace/ptrace.c

+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  *  PowerPC version
  *    Copyright (C) 1995-1996 Gary Thomas (gdt@linuxppc.org)
@@ -9,10 +10,6 @@
  *
  * Modified by Cort Dougan (cort@hq.fsmlabs.com)
  * and Paul Mackerras (paulus@samba.org).
- *
- * This file is subject to the terms and conditions of the GNU General
- * Public License.  See the file README.legal in the main directory of
- * this archive for more details.
  */
 
 #include <linux/regset.h>

arch/powerpc/kernel/signal.c

+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Common signal handling code for both 32 and 64 bits
  *
  *    Copyright (c) 2007 Benjamin Herrenschmidt, IBM Corporation
  *    Extracted from signal_32.c and signal_64.c
- *
- * This file is subject to the terms and conditions of the GNU General
- * Public License.  See the file README.legal in the main directory of
- * this archive for more details.
  */
 
 #include <linux/resume_user_mode.h>

arch/powerpc/kernel/signal.h

-/*
+/* SPDX-License-Identifier: GPL-2.0-or-later
+ *
  *    Copyright (c) 2007 Benjamin Herrenschmidt, IBM Corporation
  *    Extracted from signal_32.c and signal_64.c
- *
- * This file is subject to the terms and conditions of the GNU General
- * Public License.  See the file README.legal in the main directory of
- * this archive for more details.
  */
 
 #ifndef _POWERPC_ARCH_SIGNAL_H