ASoC: wcd9310: Introduce MBHC multi button support

So far wcd9310 driver assumes that any headset button press is headset
button 0 press.
Change driver to measure voltage when button is pressed to discriminate and
notify button's number.

In order to have better detection performance, default measurement
samples are shortened.

CRs-fixed: 327081
Change-Id: Ic0e2a5297974e1858684f0ef2a0b424141ee7464
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>