gpio: siox: explicitly support only threaded irqs

The gpio-siox driver uses handle_nested_irq() to implement its
interrupt support. This is only capable of handling threaded irq
actions. For a hardirq action it triggers a NULL pointer oops.
(It calls action->thread_fn which is NULL then.)

Prevent registration of a hardirq action by setting
gpio_irq_chip::threaded to true.

Cc: u.kleine-koenig@pengutronix.de
Fixes: be8c8fac ("gpio: new driver to work with a 8x12 siox")
Cc: stable@vger.kernel.org
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>