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>

arch/arm/dts/imx7d-sdb-u-boot.dtsi

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-2.0+ OR MIT
+
 #include "imx7s-u-boot.dtsi"
 
 &fec2 {