dt-bindings: iio: ad7192: Add mandatory reference voltage source

Add required reference voltage (VRef) supply regulator.

AD7192 requires three independent voltage sources: DVdd, AVdd and VRef
(on REFINx pin pairs).

Fixes: b581f748 ("staging: iio: adc: ad7192: move out of staging")
Signed-off-by: Fabrizio Lamarque <fl.scratchpad@gmail.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Cc: <Stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20230530075311.400686-5-fl.scratchpad@gmail.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/adc/adi,ad7192.yaml

@@ -47,6 +47,9 @@ properties:
   avdd-supply:
     description: AVdd voltage supply
 
+  vref-supply:
+    description: VRef voltage supply
+
   adi,rejection-60-Hz-enable:
     description: |
       This bit enables a notch at 60 Hz when the first notch of the sinc
@@ -89,6 +92,7 @@ required:
   - interrupts
   - dvdd-supply
   - avdd-supply
+  - vref-supply
   - spi-cpol
   - spi-cpha
 
@@ -115,6 +119,7 @@ examples:
             interrupt-parent = <&gpio>;
             dvdd-supply = <&dvdd>;
             avdd-supply = <&avdd>;
+            vref-supply = <&vref>;
 
             adi,refin2-pins-enable;
             adi,rejection-60-Hz-enable;