Additional properties in child node should not be allowed.

Prevent them adding 'additionalProperties: false'
Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20231011061437.64213-8-herve.codina@bootlin.comSigned-off-by: Rob Herring <robh@kernel.org>