dt-bindings: opp: Add missing (unevaluated|additional)Properties on child nodes

In order to ensure only documented properties are present, node schemas
must have unevaluatedProperties or additionalProperties set to false
(typically).

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>