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>