soc: Add SPDX license headers to Makefiles

Signed-off-by: Martin Roth <gaumless@gmail.com>
Change-Id: Ic875708697f07b6dae09d27dbd67eb8b960749f0
Reviewed-on: https://review.coreboot.org/c/coreboot/+/68984
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Sean Rhodes <sean@starlabs.systems>

src/soc/intel/Makefile.inc

@@ -1,3 +1,4 @@
+## SPDX-License-Identifier: GPL-2.0-only
 ifeq ($(CONFIG_STITCH_ME_BIN),y)
 
 objcse := $(obj)/cse