Stereo microphone is not supported

[gardners]

The stereo microphone is likely very easy to add support for. We can just map the left and right samples to a couple of registers.

The ADMP421 outputs in Pulse Density Modulation (PDM), which would need conversion to 8-bit samples.

The first stage is to simply clock the unit at 1MHz - 5MHz. Higher frequencies are probably preferable for improving audio quality.

Then pull the audio bits out for left and right channels alternately.

Finally, the samples need to be integrated from the pulse stream. We need to find a suitable algorithm for this. It might be as simple as counting the 1s in the most recent x pulses.