dt-bindings: memory-controllers: Make "additionalProperties: true" explicit

Make it explicit that child nodes have additional properties and the
child node schema is not complete. The complete schemas are applied
separately based the compatible strings.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Roger Quadros <rogerq@kernel.org>
Link: https://lore.kernel.org/r/20230925212626.1974676-1-robh@kernel.org


Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>