dt-bindings: serial: add missing "additionalProperties" on child nodes

All nodes need an explicit additionalProperties or unevaluatedProperties
unless a $ref has one that's false.  Add missing additionalProperties
for serial-attached devices.
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240811-dt-bindings-serial-peripheral-props-v1-1-1dba258b7492@linaro.orgSigned-off-by: Rob Herring (Arm) <robh@kernel.org>

Documentation/devicetree/bindings/serial/serial.yaml

@@ -92,6 +92,7 @@ patternProperties:
     if:
       type: object
     then:
+      additionalProperties: true
       description:
         Serial attached devices shall be a child node of the host UART device
         the slave device is attached to. It is expected that the attached