rtc: at91rm9200: add correction support

The sama5d4 and sama5d2 RTCs are able to correct for imprecise crystals, up
to 1953 ppm.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20201108232001.1580128-1-alexandre.belloni@bootlin.com