Documentation: dt: usb: atmel-usb: add unit-address

The endpoint nodes have a reg property, they also need a unit-address. Add
them in the example.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

Documentation/devicetree/bindings/usb/atmel-usb.txt

@@ -113,51 +113,51 @@ usb2: gadget@fff78000 {
 	clock-names = "hclk", "pclk";
 	atmel,vbus-gpio = <&pioB 19 0>;
 
-	ep0 {
+	ep@0 {
 		reg = <0>;
 		atmel,fifo-size = <64>;
 		atmel,nb-banks = <1>;
 	};
 
-	ep1 {
+	ep@1 {
 		reg = <1>;
 		atmel,fifo-size = <1024>;
 		atmel,nb-banks = <2>;
 		atmel,can-dma;
 		atmel,can-isoc;
 	};
 
-	ep2 {
+	ep@2 {
 		reg = <2>;
 		atmel,fifo-size = <1024>;
 		atmel,nb-banks = <2>;
 		atmel,can-dma;
 		atmel,can-isoc;
 	};
 
-	ep3 {
+	ep@3 {
 		reg = <3>;
 		atmel,fifo-size = <1024>;
 		atmel,nb-banks = <3>;
 		atmel,can-dma;
 	};
 
-	ep4 {
+	ep@4 {
 		reg = <4>;
 		atmel,fifo-size = <1024>;
 		atmel,nb-banks = <3>;
 		atmel,can-dma;
 	};
 
-	ep5 {
+	ep@5 {
 		reg = <5>;
 		atmel,fifo-size = <1024>;
 		atmel,nb-banks = <3>;
 		atmel,can-dma;
 		atmel,can-isoc;
 	};
 
-	ep6 {
+	ep@6 {
 		reg = <6>;
 		atmel,fifo-size = <1024>;
 		atmel,nb-banks = <3>;