dt-bindings: iio: st,stm32-adc: add top-level constraints

Properties with variable number of items per each device are expected to
have widest constraints in top-level "properties:" block and further
customized (narrowed) in "if:then:". Add missing top-level constraints
for clock-names.

Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Reviewed-by: Fabrice Gasnier <[e-mail]>
Acked-by: Conor Dooley <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

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

···
           It's not present on stm32f4.
           It's required on stm32h7 and stm32mp1.
 
-  clock-names: true
+  clock-names:
+    minItems: 1
+    maxItems: 2
 
   st,max-clk-rate-hz:
     description: