mfd: arizona: Remove audio bindings from MFD binding document

Now the audio bindings have their own binding document update the main
MFD document to point to that file and remove the audio 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>