soc: Add SPDX license headers to Kconfig files

Change-Id: Ie7bc4f3ae00bb9601001dbb71e7c3c84fd4f759a
Signed-off-by: Martin Roth <gaumless@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/80596
Reviewed-by: Subrata Banik <subratabanik@google.com>
Reviewed-by: Yidi Lin <yidilin@google.com>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/src/soc/intel/elkhartlake/Kconfig b/src/soc/intel/elkhartlake/Kconfig
index 3361c0d..13fa925 100644
--- a/src/soc/intel/elkhartlake/Kconfig
+++ b/src/soc/intel/elkhartlake/Kconfig
@@ -1,3 +1,5 @@
+## SPDX-License-Identifier: GPL-2.0-only
+
 config SOC_INTEL_ELKHARTLAKE
 	bool
 	select ACPI_INTEL_HARDWARE_SLEEP_VALUES