dt-bindings: atmel-usart: Add microchip,sam9x60-{usart, dbgu}

Add microchip,sam9x60-usart and add microchip,sam9x60-dbgu to DT
bindings documentation.
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/atmel-usart.txt

@@ -4,8 +4,10 @@ Required properties for USART:
 - compatible: Should be one of the following:
 	- "atmel,at91rm9200-usart"
 	- "atmel,at91sam9260-usart"
+	- "microchip,sam9x60-usart"
 	- "atmel,at91rm9200-dbgu", "atmel,at91rm9200-usart"
 	- "atmel,at91sam9260-dbgu", "atmel,at91sam9260-usart"
+	- "microchip,sam9x60-dbgu", "microchip,sam9x60-usart"
 - reg: Should contain registers location and length
 - interrupts: Should contain interrupt
 - clock-names: tuple listing input clock names.