dt-bindings: pwm: allwinner,sun4i-a10-pwm: 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: Rob Herring (Arm) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Uwe Kleine-König <[e-mail]>

Documentation/devicetree/bindings/pwm/allwinner,sun4i-a10-pwm.yaml

···
       - description: Module Clock
       - description: Bus Clock
 
-  # Even though it only applies to subschemas under the conditionals,
-  # not listing them here will trigger a warning because of the
-  # additionalsProperties set to false.
-  clock-names: true
+  clock-names:
+    minItems: 1
+    items:
+      - const: mod
+      - const: bus
 
   resets:
     maxItems: 1