dts: omap5: board-common: add phandle to reference Palmas gpadc

Will be needed for iio based drivers.
Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>

arch/arm/boot/dts/omap5-board-common.dtsi

@@ -415,7 +415,7 @@ rtc {
 			ti,backup-battery-charge-high-current;
 		};
 
-		gpadc {
+		gpadc: gpadc {
 			compatible = "ti,palmas-gpadc";
 			interrupts = <18 0
 				      16 0