rtc: at91rm9200: add sama5d4 and sama5d2 compatibles

Both the sama5d4 and sama5d2 RTCs have more features than the previous
RTCs, add a compatible string for them.

Link: https://lore.kernel.org/r/20191229204421.337612-3-alexandre.belloni@bootlin.com


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