gpio: add a userspace character device ABI

Put in some documentation for the new character device ABI
so we can properly etch it in stone.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/ABI/testing/gpio-cdev

@@ -0,0 +1,26 @@
+What:		/dev/gpiochip[0-9]+
+Date:		November 2015
+KernelVersion:	4.4
+Contact:	linux-gpio@vger.kernel.org
+Description:
+		The character device files /dev/gpiochip* are the interface
+		between GPIO chips and userspace.
+
+		The ioctl(2)-based ABI is defined and documented in
+		[include/uapi]<linux/gpio.h>.
+
+		The following file operations are supported:
+
+		open(2)
+		Currently the only useful flags are O_RDWR.
+
+		ioctl(2)
+		Initiate various actions.
+		See the inline documentation in [include/uapi]<linux/gpio.h>
+		for descriptions of all ioctls.
+
+		close(2)
+		Stops and free up the I/O contexts that was associated
+		with the file descriptor.
+
+Users:		TBD