ARM: dts: imx7s-warp: use SPDX-License-Identifier

Adopt the SPDX license identifier headers to ease
license compliance management.

Signed-off-by: Pierre-Jean Texier <texier.pj2@gmail.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>