staging: clocking-wizard: add SPDX identifier.

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 staging clocking-wizard driver 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.

This work is based on a script and data from Thomas Gleixner, Philippe
Ombredanne, and Kate Stewart.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Cc: Philippe Ombredanne <pombredanne@nexb.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/clocking-wizard/clk-xlnx-clock-wizard.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * Xilinx 'Clocking Wizard' driver
  *