Add SPDX license identifiers

Add license information to files. Mostly automated with:

    find src/ -name '*.[c,h]' | xargs sed -i '1s,^,// SPDX-License-Identifier: GPL-3.0-only\n\n,'
    find src/ -name '*.mk' | xargs sed -i '1s,^,# SPDX-License-Identifier: GPL-3.0-only\n\n,'

src/board/system76/galp3-c/board.c

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 #include <board/battery.h>
 #include <board/board.h>
 #include <board/gpio.h>

src/board/system76/galp3-c/board.mk

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: GPL-3.0-only
+
 EC=it8587e
 
 # Add keymap to src

src/board/system76/galp3-c/gpio.c

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 #include <board/gpio.h>
 #include <common/debug.h>
 

src/board/system76/galp3-c/include/board/gpio.h

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 #ifndef _BOARD_GPIO_H
 #define _BOARD_GPIO_H
 

src/board/system76/galp3-c/include/board/keymap.h

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 #ifndef _BOARD_KEYMAP_H
 #define _BOARD_KEYMAP_H
 

src/board/system76/galp3-c/kbled.c

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 #include <board/kbled.h>
 #include <common/macro.h>
 #include <ec/dac.h>

src/board/system76/galp3-c/keymap/carl.c

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 // Default layout - http://www.keyboard-layout-editor.com/#/gists/6aec6d441a039b76ec0895bd6bbda68d
 
 #include <board/keymap.h>

src/board/system76/galp3-c/keymap/default.c

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 // Default layout - http://www.keyboard-layout-editor.com/#/gists/6aec6d441a039b76ec0895bd6bbda68d
 
 #include <board/keymap.h>

src/board/system76/galp3-c/keymap/jeremy.c

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 // Jeremy's layout - http://www.keyboard-layout-editor.com/#/gists/fe00274727fc01b78a79d79473cd547c
 
 #include <board/keymap.h>