dt-bindings: iio: adc: qcom,spmi-vadc: Add scale function property

Update qcom,spmi-vadc bindings to include "qcom,scale-fn-type"
property for ADC7.

Change-Id: Ieb8ad5779b078dd77838c498b3a4e0bbcf907df8
Signed-off-by: Jishnu Prakash <quic_jprakash@quicinc.com>
[quic_amelende@quicinc.com: Converted binding change from .txt to .yaml
 and removed changes already captured in yaml]
Signed-off-by: Anjelique Melendez <quic_amelende@quicinc.com>

bindings/iio/adc/qcom,spmi-vadc.yaml

@@ -142,6 +142,14 @@ patternProperties:
             from the ADC that is an average of multiple samples. The value
             selected is 2^(value).
 
+      qcom,scale-fn-type:
+        $ref: /schemas/types.yaml#/definitions/uint32
+        description: |
+            The index of the VADC scale function used to convert raw ADC code to
+            physical scaled unites for the channel. Defined for compatibles
+            "qcom,spmi-adc5" and "qcom,spmi-adc7".
+            See include/dt-bindings/iio/qcom,spmi-vadc.h for values.
+
     required:
       - reg