rtc: mxc: use dev_pm_set_wake_irq() to simplify code

rtc: mxc: use dev_pm_set_wake_irq() to simplify code

With calling dev_pm_set_wake_irq() to set MXC 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>