Input: gpio_keys - add device tree support for interrupt only keys

This features already exists for board config setups. Add support for
device tree based systems.

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

diff --cc Documentation/devicetree/bindings/input/gpio-keys.txt
Simple merge

diff --cc drivers/input/keyboard/gpio_keys.c
Simple merge