dt-bindings: Explicitly allow additional properties in board/SoC schemas

dt-bindings: Explicitly allow additional properties in board/SoC schemas

In order to add meta-schema checks for additional/unevaluatedProperties
being present, all schema need to make this explicit. As the top-level
board/SoC schemas always have additional properties, add
'additionalProperties: true'.

Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201005183830.486085-4-robh@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>