dts: bindings: adc: stm32: add oversampler

On STM32 ADC, there are currently two types of oversamplers (if present),
one with 8 available oversampling values, the other with 1024.
To simplify the driver, add the oversampler as a dts property.
Also add defines to avoid magic values in the dtsi.

Signed-off-by: Guillaume Gautier <guillaume.gautier-ext@st.com>

dts/bindings/adc/st,stm32-adc.yaml

@@ -104,5 +104,18 @@ properties:
       - <NOT_FULLY_CONFIGURABLE>: Not fully configurable sequencer
       - <FULLY_CONFIGURABLE>: Fully configurable sequencer
 
+  st,adc-oversampler:
+    type: string
+    required: true
+    enum:
+      - "OVERSAMPLER_NONE"
+      - "OVERSAMPLER_MINIMAL"
+      - "OVERSAMPLER_EXTENDED"
+    description: |
+      Type of ADC oversampler:
+      - "OVERSAMPLER_NONE": No oversampler
+      - "OVERSAMPLER_MINIMAL": Oversampler with 8 possible oversampling values (2, 4, 8, ..., 256)
+      - "OVERSAMPLER_EXTENDED": Oversampler with 1024 possible oversampling values (1..1024)
+
 io-channel-cells:
   - input