Input: gpio_keys - add support for interrupt only keys

Some of buttons, like power-on key or onkey, may only generate interrupts
when pressed and not actually be mapped as gpio in the system. Allow
setting gpio to invalid value and specify IRQ instead to support such
keys. The debounce timer is used not to debounce but to ignore new IRQs
coming while button is kept pressed.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>