rtc: hym8563: make the irq optional

Sometimes the irq line is not connected to any soc-pin. This does
not hinder basic timekeeping functionality of the rtc, so probe
should not fail in this case.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>