Input: gpio-keys - add gpiolib debounce support

gpiolib now has debounce support added in .35, so let's make use of it.
This allows to use hardware GPIO debouncing on some platforms like OMAP.

In case gpiolib debounce setup fails for some GPIO, the driver will fall
back to timer based debouncing, which is what it used before.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>