rtc: snvs: use dev_pm_set_wake_irq() to simplify code

With calling dev_pm_set_wake_irq() to set SNVS RTC as wakeup
source for suspend, generic wake irq mechanism will automatically
enable it as wakeup source when suspend, then the suspend/resume
callback which are ONLY for enabling/disabling irq wake can be
removed, it simplifies the code.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>