mb/system76: Add SPDX ID to devicetree files

Change-Id: I55f2730f7277a3c699b86ded5864e9690d92d518
Signed-off-by: Tim Crawford <tcrawford@system76.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/82700
Reviewed-by: Jeremy Soller <jeremy@system76.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/system76/adl/devicetree.cb

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: GPL-2.0-only
+
 chip soc/intel/alderlake
 	register "common_soc_config" = "{
 		// Touchpad I2C bus