dt-bindings: soundwire: fix example

dt-bindings: soundwire: fix example

As wsa881x schema mentions #sound-dai-cells as required property,
so update soundwire-controller.yaml example so that dt_bindings_check
does not fail as below:

Documentation/devicetree/bindings/soundwire/soundwire-controller.example.dt.yaml:
speaker@0,1: '#sound-dai-cells' is a required property
Documentation/devicetree/bindings/soundwire/soundwire-controller.example.dt.yaml:
speaker@0,2: '#sound-dai-cells' is a required property

Reported-by: Rob Herring <robh@kernel.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20200114094806.15846-1-srinivas.kandagatla@linaro.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>