ARM: at91/dt: sama5d4: fix external trigger property

The property is actually atmel,adc-use-external-triggers.
Signed-off-by: NAlexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: NNicolas Ferre <nicolas.ferre@atmel.com>

arch/arm/boot/dts/sama5d4.dtsi

@@ -1207,7 +1207,7 @@
 				clock-names = "adc_clk", "adc_op_clk";
 				atmel,adc-channels-used = <0x01f>;
 				atmel,adc-startup-time = <40>;
-				atmel,adc-use-external;
+				atmel,adc-use-external-triggers;
 				atmel,adc-vref = <3000>;
 				atmel,adc-res = <8 10>;
 				atmel,adc-sample-hold-time = <11>;