ASoC: DMIC codec - Add input widget

Digital microphones can have some additional elements in their
audio path (like microphone bias). An input widget is required
for digital microphone CODEC driver to allow external connections
in machine drivers.

Signed-off-by: Misael Lopez Cruz <misael.lopez@ti.com>
Signed-off-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>