dt-bindings: rtc: at91rm9200: add sama7g5 compatible

Add compatible for SAMA7G5 RTC. At the moment the driver is falling
back on SAM9X60's compatible but SAMA7G5 doesn't have the tamper mode
register and tamper debounce period register thus the need for a new
compatible to differentiate b/w these two in case tamper feature will
be implemented in future.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/1610016372-31784-1-git-send-email-claudiu.beznea@microchip.com