platform/x86: intel_punit_ipc: Convert to use SPDX identifier

Reduce size of duplicated comments by switching to use SPDX identifier.

No functional change.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

drivers/platform/x86/intel_punit_ipc.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * Driver for the Intel P-Unit Mailbox IPC mechanism
  *
  * (C) Copyright 2015 Intel Corporation
  *
- * 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.
- *
  * The heart of the P-Unit is the Foxton microcontroller and its firmware,
  * which provide mailbox interface for power management usage.
  */