dt-bindings: atmel-can: add microchip,sam9x60-can

Add microchip,sam9x60-can to DT bindings documentation.
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1578673089-3484-4-git-send-email-claudiu.beznea@microchip.comSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Documentation/devicetree/bindings/net/can/atmel-can.txt

 * AT91 CAN *
 
 Required properties:
-  - compatible: Should be "atmel,at91sam9263-can" or "atmel,at91sam9x5-can"
+  - compatible: Should be "atmel,at91sam9263-can", "atmel,at91sam9x5-can" or
+    "microchip,sam9x60-can"
   - reg: Should contain CAN controller registers location and length
   - interrupts: Should contain IRQ line for the CAN controller