dt-bindings: iio: adc: ti,tsc2046: add vref-supply property

Add property for the voltage reference supply.
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220904100203.3614502-1-o.rempel@pengutronix.deSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/adc/ti,tsc2046.yaml

@@ -23,6 +23,9 @@ properties:
   interrupts:
     maxItems: 1
 
+  vref-supply:
+    description: Optional supply of the reference voltage
+
   "#io-channel-cells":
     const: 1