rtc: sa1100: make alarms useful

Currently, the driver unregisters the IRQs when the rtc character device is
closed. This means that the device needs to stay open to get alarms while
the usual use case will open the device, set the alarm and close the
device.

Move the IRQ requests to sa1100_rtc_probe() and use the devm managed
versions so we don't need to free them.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>