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/google/chromeec/Makefile.inc

@@ -1,3 +1,4 @@
+## SPDX-License-Identifier: GPL-2.0-only
 ifeq ($(CONFIG_EC_GOOGLE_CHROMEEC),y)
 
 subdirs-y += audio_codec

src/ec/google/chromeec/audio_codec/Makefile.inc

@@ -1 +1,2 @@
+## SPDX-License-Identifier: GPL-2.0-only
 ramstage-$(CONFIG_EC_GOOGLE_CHROMEEC_AUDIO_CODEC) += audio_codec.c

src/ec/google/chromeec/i2c_tunnel/Makefile.inc

@@ -1 +1,2 @@
+## SPDX-License-Identifier: GPL-2.0-only
 ramstage-$(CONFIG_EC_GOOGLE_CHROMEEC_I2C_TUNNEL) += i2c_tunnel.c

src/ec/google/chromeec/mux/Makefile.inc

@@ -1,2 +1,3 @@
+## SPDX-License-Identifier: GPL-2.0-only
 subdirs-y += conn
 ramstage-$(CONFIG_EC_GOOGLE_CHROMEEC_MUX) += mux.c

src/ec/google/chromeec/mux/conn/Makefile.inc

@@ -1 +1,2 @@
+## SPDX-License-Identifier: GPL-2.0-only
 ramstage-$(CONFIG_EC_GOOGLE_CHROMEEC_MUX) += conn.c