wm8962: support detecting an external analogue microphone

We currently only detect headphones, not the mic (in case of a headset). So our ucm profile that makes the headset mic available is wrong because it listens to the headphones jack, not whether a MIC is available (IN3R is grounded, see https://source.puri.sm/Librem5/linux-next/-/issues/5#note_111192 )

added audio label

mentioned in issue #5 (closed)

mentioned in issue user-personas#9

So the way we'll be able to do this will probably be to try detection (by zero-crossing ?) in the event handler that runs after headphones are plugged in.

mentioned in merge request librem5-base!303 (closed)

@martin.kepplinger I think what you're looking for is MICBIAS CURRENT DETECT in the reference manual.