ASoC: wcd9xxx: introduce headphone impedance detection support

Different headphones have different impedance that results in volume level
difference on ears as lower impedance headphone will have higher
volume with the same gain on PAs.
Provide impedance of headphones to be able to adjust gain according to
impedance so user can have consistent volume level with different
headphones.

Change-Id: I707f34344cd0068507bdd8a0192eb7afea1147f2
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>