gpio: ABI: mark the sysfs ABI as obsolete

This marks the (optional) sysfs GPIO ABI as obsolete and schedules
it for removal in 2020.

Cc: Johan Hovold <johan@kernel.org>
Cc: Michael Welling <mwelling@ieee.org>
Cc: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/ABI/testing/sysfs-gpio → Documentation/ABI/obsolete/sysfs-gpio

 What:		/sys/class/gpio/
 Date:		July 2008
 KernelVersion:	2.6.27
-Contact:	David Brownell <dbrownell@users.sourceforge.net>
+Contact:	Linus Walleij <linusw@kernel.org>
 Description:
 
   As a Kconfig option, individual GPIO signals may be accessed from
@@ -26,3 +26,5 @@ Description:
 	    /label ... (r/o) descriptive, not necessarily unique
 	    /ngpio ... (r/o) number of GPIOs; numbered N to N + (ngpio - 1)
 
+  This ABI is deprecated and will be removed after 2020. It is
+  replaced with the GPIO character device.

MAINTAINERS

@@ -4816,6 +4816,7 @@ 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:	Documentation/ABI/obsolete/sysfs-gpio
 F:	drivers/gpio/
 F:	include/linux/gpio/
 F:	include/linux/gpio.h