ASoC: max98090: add digital mic mux to record path

The max98090 driver currently treats the digital mic enable as a supply
on the record path, causing the digital mic enable to always be turned on
when attempting to record.  This is incorrect, however, since the digital
mic enable is also a mux control where 0 selects the ADC output as input
to the record-path DSP and 1 selects the digital mic.  This patch adds
a virtual DMIC mux to the reocrd path so that we can switch between the
ADC and the digital mic for recording.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>