Replace the license text with SPDX identifiers.

Avoid the following checkpatch warning:

arch/arm/boot/dts/imx6qdl-sabrelite.dtsi:1: warning: Missing or
 malformed SPDX-License-Identifier tag in line 1
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>