src: Add missing SPDX license headers

Other files in the commits that added these files were licensed under
GPL-2.0-only, and the project as a whole is GPL-2.0-only, so use that
as the license.

Change-Id: I6c1a7ba582f61f98069ebf3857a8b5bdc8588c3e
Signed-off-by: Nicholas Chin <nic.c3.14@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/81421
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/google/myst/variants/baseboard/include/baseboard/port_descriptors.h

@@ -1,3 +1,5 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
 #ifndef __BASEBOARD_PORT_DESCRIPTORS_H__
 #define __BASEBOARD_PORT_DESCRIPTORS_H__
 

src/mainboard/google/skyrim/variants/baseboard/include/baseboard/port_descriptors.h

@@ -1,3 +1,5 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
 #ifndef __BASEBOARD_PORT_DESCRIPTORS_H__
 #define __BASEBOARD_PORT_DESCRIPTORS_H__
 

src/soc/sifive/fu740/include/soc/gpio.h

@@ -1,3 +1,5 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
 #ifndef _SOC_SIFIVE_FU740_GPIO_H_
 #define _SOC_SIFIVE_FU740_GPIO_H_