ASoc: wcd: mbhc: Add key code and linein assign function

Add key code assignment function in mbhc v2 driver.
Need to assign keycode from machine driver if required.
Add linein threshold assignment function in mbhc v2 driver.
Need to assign linein threshold from machine driver if required.

CRs-Fixed: 847638
Change-Id: Id770bd3c15703da23db727fbdbb8fffe20435747
Signed-off-by: Walter Yang <yandongy@codeaurora.org>