arm: dts: zynq-coraz7s-ad7687-pmdz: Set bipolar prop for diff channel

Pulsar devices do differential voltage measurement by default. When the
negative input is restricted to a known voltage level, the ADC channel can
be configured to do unipolar measurement in which case IN+ is not allowed
to go below IN- and, except for possible offset, all measurements should
then express a positive quantity. In contrast, when the negative input is
not restricted to a fixed level, the measurement can express positive
(IN+ > IN-) and negative (IN+ < IN-) quantities.

Set the bipolar property to the differential channel for devices which
default chip configuration does not imply any fixed voltage to IN-.

Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>

Author: machschmitt

arch/arm/boot/dts/zynq-coraz7s-ad7687-pmdz.dts

@@ -61,6 +61,7 @@
 			vref-supply = <&vref>;
 			channel@0 {
 				reg = <0>;
+				bipolar;
 				diff-channels = <0 1>;
 			};
 		};

arch/arm/boot/dts/zynq-coraz7s-ad7984.dts

@@ -61,6 +61,7 @@
 			vref-supply = <&vref>;
 			channel@0 {
 				reg = <0>;
+				bipolar;
 				diff-channels = <0 1>;
 			};
 		};