ASoC: sn95031: Add support for reading mic bias

This patch adds support to read the mic bias voltage
when a jack is inserted. It uses ADC to measure.

Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Harsha Priya <priya.harsha@intel.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>