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 <marcel.ziswiler@toradex.com>
Reviewed-by: Abel Vesa <abel.vesa@linaro.org>
Signed-off-by: Abel Vesa <abel.vesa@linaro.org>
Link: https://lore.kernel.org/r/20230119085421.102804-2-marcel@ziswiler.com