ASoC: ak4613: Undo conversion to YAML bindings

Revert 5b235b55

 ("ASoC: dt-bindings: ak4613: switch to yaml base
Documentation") for the time being since it depends on other conversions.

Reported-by: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>