Input: gpio_mouse - add DT bindings

This adds DT bindings for simple mice attached to GPIO lines. As the
properties are very general and pertains to all mice I can think of, we use
very generic names for the 4-7 GPIO lines, "up", "down" etc.

Acked-by: Hans-Christian Noren Egtvedt <egtvedt@samfundet.no>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>