Add SDCA class driver

Add SDCA class driver

Merge series from Charles Keepax <ckeepax@opensource.cirrus.com>:

This series adds an initial SDCA class driver, this consists of a
primary driver attached to the SoundWire device, and auxiliary drivers
representing each of the functions of the SDCA device. These drivers all
use the APIs added over the past series's to provide the class
functionality, as such these final drivers themselves are quite thin.

Also a few fix ups at the start of the series that have gathered up
whilst the last SDCA series was in review.