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: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>