ASoC: mxs-sgtl5000: add audio-routing support

Add dapm_widgets to machine-driver (from imx-sgtl5000).
If the "audio-routing"-property is present at probing the dapm-widgets
getting linked to the card.

Signed-off-by: Christian Fischer <fischerc@swissphone.de>
Signed-off-by: Mark Brown <broonie@kernel.org>