dt-bindings: rtc: at91: rename rtt bindings file

atmel,at91sam9-rtc is a confusing name for this file as it is documenting
the RTT used as an RTC and not the other regular RTC (atmel,at91rm9200-rtc
and atmel,at91sam9x5-rtc)

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20220308155735.54146-1-alexandre.belloni@bootlin.com