dts: bindings: adc: stm32: use string enum instead of number enum

STM32 sequencer property and clock source were defined using
arbitrary numbers. Use string instead.

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

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

@@ -22,15 +22,15 @@ properties:
     const: 1
 
   st,adc-clock-source:
-    type: int
+    type: string
     required: true
     enum:
-      - 1 # SYNC for synchronous ADC clock source
-      - 2 # ASYNC for asynchronous ADC clock source
+      - "SYNC"
+      - "ASYNC"
     description: |
       Type of ADC clock source :
-      - <SYNC>: derived from the bus clock.
-      - <ASYNC> : independent and asynchronous with the bus clock
+      - "SYNC": derived from the bus clock.
+      - "ASYNC" : independent and asynchronous with the bus clock
       One of the two values may not apply to some series. Refer to the RefMan.
       If an asynchronous clock is selected, a domain clock in the clock property
       has to be defined explicitly.
@@ -94,15 +94,15 @@ properties:
       Number of sampling time common channels for this ADC instance, if any.
 
   st,adc-sequencer:
-    type: int
+    type: string
     required: true
     enum:
-      - 0 # NOT_FULLY_CONFIGURABLE
-      - 1 # FULLY_CONFIGURABLE
+      - "NOT_FULLY_CONFIGURABLE"
+      - "FULLY_CONFIGURABLE"
     description: |
       Type of ADC sequencer:
-      - <NOT_FULLY_CONFIGURABLE>: Not fully configurable sequencer
-      - <FULLY_CONFIGURABLE>: Fully configurable sequencer
+      - "NOT_FULLY_CONFIGURABLE": Not fully configurable sequencer
+      - "FULLY_CONFIGURABLE": Fully configurable sequencer
 
   st,adc-oversampler:
     type: string