ASoC: wcd9306: Add codec controls to get impedance of hphr and hphl

Add kcontrol to get the impedance of headphone left and headphone
right. These controls are applicable to both tapan codec versions
wcd9306 and wcd9302.

Change-Id: Ia414029f781c151084a9ad93795ef8a1aa4c95a2
Signed-off-by: Phani Kumar Uppalapati <phaniu@codeaurora.org>