staging: gasket: add SPDX identifiers to all files.

It's good to have SPDX identifiers in all files to make it easier to
audit the kernel tree for correct licenses.

Fix up the all of the staging gasket files to have a proper SPDX
identifier, based on the license text in the file itself.  The SPDX
identifier is a legally binding shorthand, which can be used instead of
the full boiler plate text.

Cc: Rob Springer <rspringer@google.com>
Cc: John Joseph <jnjoseph@google.com>
Cc: Ben Chan <benchan@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/gasket/apex.h

+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * Apex kernel-userspace interface definition(s).
  *

drivers/staging/gasket/apex_driver.c

+// SPDX-License-Identifier: GPL-2.0
 /* Driver for the Apex chip.
  *
  * Copyright (C) 2018 Google, Inc.

drivers/staging/gasket/gasket.h

+/* SPDX-License-Identifier: GPL-2.0 */
 /* Common Gasket device kernel and user space declarations.
  *
  * Copyright (C) 2018 Google, Inc.

drivers/staging/gasket/gasket_constants.h

+/* SPDX-License-Identifier: GPL-2.0 */
 /* Copyright (C) 2018 Google, Inc.
  *
  * This software is licensed under the terms of the GNU General Public

drivers/staging/gasket/gasket_core.c

+// SPDX-License-Identifier: GPL-2.0
 /* Gasket generic driver framework. This file contains the implementation
  * for the Gasket generic driver framework - the functionality that is common
  * across Gasket devices.

drivers/staging/gasket/gasket_core.h

+/* SPDX-License-Identifier: GPL-2.0 */
 /* Gasket generic driver. Defines the set of data types and functions necessary
  * to define a driver using the Gasket generic driver framework.
  *

drivers/staging/gasket/gasket_interrupt.c

+// SPDX-License-Identifier: GPL-2.0
 /* Copyright (C) 2018 Google, Inc.
  *
  * This software is licensed under the terms of the GNU General Public

drivers/staging/gasket/gasket_interrupt.h

+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * Gasket common interrupt module. Defines functions for enabling
  * eventfd-triggered interrupts between a Gasket device and a host process.

drivers/staging/gasket/gasket_ioctl.c

+// SPDX-License-Identifier: GPL-2.0
 /* Copyright (C) 2018 Google, Inc.
  *
  * This software is licensed under the terms of the GNU General Public

drivers/staging/gasket/gasket_ioctl.h

+/* SPDX-License-Identifier: GPL-2.0 */
 /* Copyright (C) 2018 Google, Inc.
  *
  * This software is licensed under the terms of the GNU General Public

drivers/staging/gasket/gasket_logging.h

+/* SPDX-License-Identifier: GPL-2.0 */
 /* Common logging utilities for the Gasket driver framework.
  *
  * Copyright (C) 2018 Google, Inc.

drivers/staging/gasket/gasket_page_table.c

+// SPDX-License-Identifier: GPL-2.0
 /* Implementation of Gasket page table support.
  *
  * Copyright (C) 2018 Google, Inc.

drivers/staging/gasket/gasket_page_table.h

+/* SPDX-License-Identifier: GPL-2.0 */
 /* Gasket Page Table functionality. This file describes the address
  * translation/paging functionality supported by the Gasket driver framework.
  * As much as possible, internal details are hidden to simplify use -

drivers/staging/gasket/gasket_sysfs.c

+// SPDX-License-Identifier: GPL-2.0
 /* Copyright (C) 2018 Google, Inc.
  *
  * This software is licensed under the terms of the GNU General Public

drivers/staging/gasket/gasket_sysfs.h

+/* SPDX-License-Identifier: GPL-2.0 */
 /* Set of common sysfs utilities.
  *
  * Copyright (C) 2018 Google, Inc.