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>
diff --git a/MAINTAINERS b/MAINTAINERS
index 4d3d1b8..a85cd6d 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -4815,6 +4815,7 @@
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/linusw/linux-gpio.git
 S:	Maintained
 F:	Documentation/gpio/
+F:	Documentation/ABI/testing/gpio-cdev
 F:	drivers/gpio/
 F:	include/linux/gpio/
 F:	include/linux/gpio.h