spi: dt-bindings: 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>
Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://lore.kernel.org/r/20230925212614.1974243-1-robh@kernel.org


Signed-off-by: Mark Brown <broonie@kernel.org>