ARM: dts: imx6sll: Fix the SPDX notation for header file

Per Documentation/process/license-rules.rst the SPDX notation for
header file should be /* */ style, so fix it accordingly.

Fixes: 9f30b6b1 ("ARM: dts: imx: Add basic dtsi file for imx6sll")
Signed-off-by: NFabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: NShawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx6sll-pinfunc.h

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