dt-bindings: ufs: qcom,ufs: document required-opps

UFS device node on SC8280XP uses required-opps:

  sc8280xp-crd.dtb: ufs@1d84000: Unevaluated properties are not allowed ('required-opps' was unexpected)

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221228124331.258416-1-krzysztof.kozlowski@linaro.org


Signed-off-by: Rob Herring <robh@kernel.org>