firmware: arm_scmi: Use the correct style for SPDX License Identifier

Fix to correct the SPDX License Identifier style in header file related
to firmware frivers for ARM SCMI message protocol.

For C header files Documentation/process/license-rules.rst mandates
C-like comments(opposed to C source files where C++ style should be
used).

While at it, change GPL-2.0 to GPL-2.0-only similar to the ones in
psci.h and scpi_protocol.h
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

include/linux/scmi_protocol.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0-only */
 /*
  * SCMI Message Protocol driver header
  *