rtc: at91rm9200: Add sam9x60 compatible

Handle the sam9x60 RTC. While it can work with the at91sam9x5 fallback, it
has crystal correction support and doesn't need to shadow IMR.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20201117133920.1229679-1-alexandre.belloni@bootlin.com