ASoC: max8960: add bindings for the max9860 codec

This adds the device tree binding documentation for the Maxim Integrated
MAX9860 mono audio voice codec.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Mark Brown <broonie@kernel.org>