rtc: at91rm9200: remove race condition

While highly unlikely, it is possible to get an interrupt as soon as it is
requested. In that case, at91_rtc_interrupt() will be called with rtc ==
NULL.

Solve that by using devm_rtc_allocate_device/rtc_register_device.

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