dt-bindings: iio: stm32-adc: add nvmem support for vrefint internal channel

Add support of nvmem. This allows to retrieve calibration data from OTP
for vrefint internal channel.
Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Reviewed-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211014131228.4692-3-olivier.moysan@foss.st.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/adc/st,stm32-adc.yaml

@@ -301,6 +301,14 @@ patternProperties:
         $ref: /schemas/types.yaml#/definitions/uint32-array
         deprecated: true
 
+      nvmem-cells:
+        items:
+          - description: Phandle to the calibration vrefint data provided by otp
+
+      nvmem-cell-names:
+        items:
+          - const: vrefint
+
     patternProperties:
       "^channel@([0-9]|1[0-9])$":
         type: object