ASoC: dt: Allow Aux Codecs to be specified using DT

This patch adds support for specifying auxiliary codecs and
codec configuration via device tree phandles.

This change adds new fields to snd_soc_aux_dev and snd_soc_codec_conf
and adds support for the changes to SoC core methods.

Signed-off-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Mark Brown <broonie@linaro.org>