ARM: dts: imx6-sabrelite: change to use SPDX identifiers

Replace the license text with SPDX identifiers.

Avoid the following checkpatch warning:

arch/arm/boot/dts/imx6q-sabrelite.dts: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>