ASoC: wcd9xxx: Add support to compute hph L & R impedance

Add support to compute impedance of the headphone left
and right on Tapan codec. This can be used to adjust gain
according to the value of impedance so that user can have
consistent volume level with different headphones.

Change-Id: Id187bbfaf20c428d367bffa77538182554917e09
Signed-off-by: Phani Kumar Uppalapati <phaniu@codeaurora.org>