ARM: dts: at91: Use the generic "rtc" node name for the rtt IPs

ARM: dts: at91: Use the generic "rtc" node name for the rtt IPs

As the DT specification recommends, the node names should be of a
generic nature. Thus, the most appropriate generic node name for
the at91 rtt IPs is the "rtc" node name.

Signed-off-by: Sergiu Moga <sergiu.moga@microchip.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/all/20220304161159.147784-3-sergiu.moga@microchip.com
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>