drivers: adc: implement voltage biasing for ADS114s08

Implement voltage biasing on the inputs of the ADC ADS114s08.

Signed-off-by: Benedikt Schmidt <benedikt.schmidt@embedded-solutions.at>

dts/bindings/adc/adc-controller.yaml

@@ -160,3 +160,10 @@ child-binding:
         interpretation depends on the driver. This is used only for drivers
         which select the ADC_CONFIGURABLE_EXCITATION_CURRENT_SOURCE_PIN
         Kconfig option.
+
+    zephyr,vbias-pins:
+      type: int
+      description: |
+        Output pin selection for the bias voltage. The actual interpretation
+        depends on the driver. This is used only for drivers which select
+        the ADC_CONFIGURABLE_VBIAS_PIN Kconfig option.

dts/bindings/adc/ti,ads114s08.yaml

@@ -45,5 +45,14 @@ properties:
     description: |
       IDAC current in microampere, the default value turns the current source off
 
+  vbias-level:
+    type: int
+    enum:
+      - 0
+      - 1
+    default: 0
+    description: |
+      bias voltage level: 0 - (AVDD+AVSS)/2, 1 - (AVDD+AVSS)/12
+
 io-channel-cells:
   - input