dt-bindings: soundwire: Add reference to soundwire-controller.yaml schema

The soundwire-controller.yaml schema already defines the form for devices
in child nodes, so there's no need to do the same in the QCom controller
binding. Add a $ref to the soundwire-controller.yaml schema and drop the
child node schema.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20231016155537.2973625-1-robh@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>