ARM: dts: at91: sama7g5: add rtc node

Add RTC node.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
[claudiu.beznea: add sama7g5 compatible as the IP has 2 extra registers
 compared with sam9x60]
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20211020094656.3343242-2-claudiu.beznea@microchip.com