Add SDCA register map support

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

This series is the next step of adding SDCA support. Here we add
helper functions to allow drivers to easily use the SDCA DisCo
information to create a register map for the device.

The basic idea here is the code takes the list of SDCA controls parsed
from DisCo and uses primarily the Access Mode to determine if the
register should be marked as readable/writable etc. Further more
some additional concepts such as DisCo Constants and Defaults are
handled. There is some potential confusion, as DisCo Constants are
handled as an entry in the regmap defaults table, whereas a DisCo
Default is simply handled as a write to the register. Alas the naming
confusion is an unavoidable result of the slight impedance mismatch
between the two systems.