ASoC: adau1701: add support for pin muxing

The ADAU1701 has 12 pins that can be configured depending on the system
configuration. Allow settting the corresponding registers from DT.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>