dt-bindings: mfd: motorola-cpcap: Document audio-codec

This adds the DT binding for the audio-codec sub-module found
inside the Motorola CPCAP PMIC.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Acked-by: Mark Brown <broonie@kernel.org>
Acked-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>