imx7d-sdb-u-boot: Pass SPDX-License tag

SPDX-License tag is missing and checkpatch complains about it.

Add the SPDX-License tag using the same one from imx7d-sdb.dts.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Marek Vasut <marex@denx.de>