dts: bindings: adc: stm32: update adc bindings doc

Update ADC bindings documentation to state that a domain clock is now
necessary if asynchronous clock is selected.

Signed-off-by: Guillaume Gautier <[email protected]>

Author: gautierg-st

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

@@ -32,6 +32,8 @@ properties:
       - <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.
 
   st,adc-prescaler:
     type: int