Input: gpio_keys - switch to using threaded IRQs

Use a threaded interrupt handler in order to permit the handler to use
a GPIO driver that causes things like I2C transactions being done inside
the handler context.

Signed-off-by: David Jander <david@protonic.nl>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>