clk: imx6sll: add proper spdx license identifier

This fixes the following error:

include/dt-bindings/clock/imx6sll-clock.h:1: warning: Improper SPDX
comment style for 'include/dt-bindings/clock/imx6sll-clock.h', please
use '/*' instead
include/dt-bindings/clock/imx6sll-clock.h:1: warning: Missing or
malformed SPDX-License-Identifier tag in line 1

Signed-off-by: Marcel Ziswiler <[e-mail]>
Reviewed-by: Abel Vesa <[e-mail]>
Signed-off-by: Abel Vesa <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

include/dt-bindings/clock/imx6sll-clock.h

···
-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * Copyright (C) 2016 Freescale Semiconductor, Inc.
  * Copyright 2017-2018 NXP.