rtc: ds3232: fix issue when irq is shared several devices

ds3232-core requests irq with IRQF_SHARED, so irq can be shared by
several devices.  But the irq handler for ds3232 unconditionally
disables the irq at first and the irq is re-enabled only when the
interrupt source was the ds3232's alarm.  This behaviour breaks the
devices sharing the same irq in the various scenarios.

This converts to use threaded irq and remove outdated code in
suspend/resume paths.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Suggested-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>