ASoC: wcd9320: Allow micbias 2 sharing

Currently wcd9320 driver assumes micbias 2 is dedicated to headset and ANC
microphones on headset.  Allow different microphones share the micbias 2 to
support hardware that have shared micbias configuration.

If micbias 2 is shared by mic biases other than headset mic, MBHC's ext
cable feature will be compromized and also headset removal can make more
pop noise.  If hardware has dedicated micbias 2 for headset, it's
recommanded to disable this feature.

Change-Id: I5d5e5c2feb406212dad6d211f49de49e64e1a888
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>