ARM: at91: at91sam9x5 RTC is not compatible with at91rm9200 one

Due to a bug with RTC IMR, we cannot consider at91sam9x5 RTC compatible
with the previous one. Modify DT compatibility string, even if the driver
is not yet modified to take it into account.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>