rtc: at91rm9200: fix reading alarm value

When alarm value is read at boot time, at91_alarm_year is not yet set to
the proper value so the year is always set to 1900.

This results in that kind of message at boot:
rtc rtc0: invalid alarm value: 1900-1-14 2:11:39

There is no way to recover from that as the alarm is now only read when
booting.

Instead, rely on the rtc core to figure out the proper year.

Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>