ASoC: arizona: Add audio device tree bindings

In keeping with the style of the rest of this drivers device tree
bindings split the audio device tree bindings into their own document.
It should be noted this patch makes no change to the binding itself
just moves the documentation into a specific file for the audio
binding. For easy of merging a separate patch removes the current
documentation from the MFD binding documentation.

Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>