Input: gpio_mouse - add device tree probing

This makes the GPIO mouse probe nicely from the device tree if found in a
tree. As the driver uses device properties it can easily be amended to also
probe from ACPI devices.

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