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: 3cb16ad6 ("dt-bindings: opp: accept array of frequencies")
Suggested-by: Rob Herring <robh@kernel.org>
Signed-off-by: David Heidelberg <david@ixit.cz>
Reviewed-by: Dhruva Gole <d-gole@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>