Documentation: dt: iio: at91_adc: fix trigger node names

The trigger doesn't need the reg property. When it is not defined, the node
name doesn't need a unit-address. Remove them from the example.
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

Documentation/devicetree/bindings/iio/adc/at91_adc.txt

@@ -59,28 +59,24 @@ adc0: adc@fffb0000 {
 	atmel,adc-res-names = "lowres", "highres";
 	atmel,adc-use-res = "lowres";
 
-	trigger@0 {
-		reg = <0>;
+	trigger0 {
 		trigger-name = "external-rising";
 		trigger-value = <0x1>;
 		trigger-external;
 	};
-	trigger@1 {
-		reg = <1>;
+	trigger1 {
 		trigger-name = "external-falling";
 		trigger-value = <0x2>;
 		trigger-external;
 	};
 
-	trigger@2 {
-		reg = <2>;
+	trigger2 {
 		trigger-name = "external-any";
 		trigger-value = <0x3>;
 		trigger-external;
 	};
 
-	trigger@3 {
-		reg = <3>;
+	trigger3 {
 		trigger-name = "continuous";
 		trigger-value = <0x6>;
 	};