Input: gpio_keys_polled - add support for abs/rel axis

Input: gpio_keys_polled - add support for abs/rel axis

This changAdd support for EV_ABS / EV_REL events to the gpio-keys-polled driver.

The driver already allows specifying what type of events (key / rel / abs)
a button generates when pressed, but for rel / abs axis we also need to
specify which value this specific gpio represents.

One use case is digital joysticks / direction-pads which are hooked up to
gpio, in this case we've left and right buttons which we want to map to
EV_ABS, ABS_X and we want generate events for left with a value of -1 and
for right with a value of +1 (and similar for up / down and ABS_Y).

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>