rtc: at91rm9200: clear RTC alarm status flag prior to suspending

As said in the SAMA5D2 datasheet, "Prior to instructing the device
to enter ULP mode 1, ... and the internal sources of wake-up must
be cleared."

This patch is to clear the RTC alarm status flag prior to suspending
to avoid the erroneous wake-up activity, as it is often used as
the wake-up source for the ULP mode 1.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>