dt-bindings: opp: drop maxItems from inner items

With recent changes within matrix dimensions calculation,
dropping maxItems: 1 provides a warning-free run.

Fixes warning such as:
arch/arm64/boot/dts/qcom/sdm845-oneplus-enchilada.dtb: opp-table: opp-200000000:opp-hz:0: [200000000, 0, 0, 150000000, 0, 0, 0, 0, 300000000] is too long

Fixes: 3cb16ad69bef ("dt-bindings: opp: accept array of frequencies")
Suggested-by: Rob Herring <[e-mail]>
Signed-off-by: David Heidelberg <[e-mail]>
Reviewed-by: Dhruva Gole <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Viresh Kumar <[e-mail]>

Documentation/devicetree/bindings/opp/opp-v2-base.yaml

···
           specific binding.
         minItems: 1
         maxItems: 32
-        items:
-          maxItems: 1
 
       opp-microvolt:
         description: |