gpio: Use SPDX header for core library

Use the SPDX headers and cut down on boilerplate to indicate the
license in the core gpiolib implementation.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
index 03d15d8..1ffd3fb 100644
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0
 #include <linux/bitmap.h>
 #include <linux/kernel.h>
 #include <linux/module.h>