staging: octeon: Add the license identifier

This patch fixes the checkpatch.pl warning:
WARNING: Missing or malformed SPDX-License-Identifier tag

Add a the SPDX-License-Identifier tag on line 1

Other files in this folder have GPL-2.0 license. So this file should
have the same license which was missing before and checkpatch.pl was
giving the warning.
Signed-off-by: Muhammad Usama Anjum <musamaanjum@gmail.com>
Link: https://lore.kernel.org/r/20200723180723.GA30699@musamaanjumSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/octeon/octeon-stubs.h

+/* SPDX-License-Identifier: GPL-2.0 */
 #define CONFIG_CAVIUM_OCTEON_CVMSEG_SIZE	512
 
 #ifndef XKPHYS_TO_PHYS