dt-bindings: rtc: add microchip,sam9x60-rtt

Add microchip,sam9x60-rtt to compatible list.
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1591779936-18577-3-git-send-email-claudiu.beznea@microchip.com

Documentation/devicetree/bindings/rtc/atmel,at91sam9-rtc.txt

 Atmel AT91SAM9260 Real Time Timer
 
 Required properties:
-- compatible: should be: "atmel,at91sam9260-rtt"
+- compatible: should be one of the following:
+	- "atmel,at91sam9260-rtt"
+	- "microchip,sam9x60-rtt", "atmel,at91sam9260-rtt"
 - reg: should encode the memory region of the RTT controller
 - interrupts: rtt alarm/event interrupt
 - clocks: should contain the 32 KHz slow clk that will drive the RTT block.