ec: Add SPDX license headers to Makefiles

Signed-off-by: Martin Roth <gaumless@gmail.com>
Change-Id: Ie5355e05982b372ef69515cfa081e2afbc7b09fe
Reviewed-on: https://review.coreboot.org/c/coreboot/+/68981
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Caveh Jalali <caveh@chromium.org>
Reviewed-by: Matt DeVillier <matt.devillier@amd.corp-partner.google.com>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>

src/ec/quanta/ene_kb3940q/Makefile.inc

@@ -1,3 +1,4 @@
+## SPDX-License-Identifier: GPL-2.0-only
 ifeq ($(CONFIG_EC_QUANTA_ENE_KB3940Q),y)
 
 ramstage-y += ec.c

src/ec/quanta/it8518/Makefile.inc

@@ -1,3 +1,4 @@
+## SPDX-License-Identifier: GPL-2.0-only
 ifeq ($(CONFIG_EC_QUANTA_IT8518),y)
 
 romstage-y += ec.c