ARM: dts: at91: sam9x60_curiosity: Add mandatory dt property for RTT

ARM: dts: at91: sam9x60_curiosity: Add mandatory dt property for RTT

atmel,rtt-rtc-time-reg is a mandatory property and encodes the GPBR
register used to store the time base when the RTT is used as an RTC.
Align the RTT with what's currently done for sam9x60ek and sama7g5ek,
and enable it by default even if RTC is also enabled.

Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>
[nicolas.ferre@microchip.com: adapt to newer kernel]
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20230928143644.208515-1-nicolas.ferre@microchip.com
Signed-off-by: Claudiu Beznea <claudiu.beznea@tuxon.dev>