rtc: rx8025: fix irq handler registration

When IRQ line for this chips is connected, devm_request_threaded_irq()
refuses to register irq handler with the following message.

genirq: Threaded irq requested with handler=NULL and !ONESHOT for irq

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Wolfgang Grandegger <wg@grandegger.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>