One possible objection is that this code could have been handled with regmap-sdw.c. However this is a new spec addition not handled by every SoundWire 1.1 and non-SDCA device, so there's no reason to load code that will never be used.
Also in practice it's extremely unlikely that CONFIG_REGMAP would not be selected with CONFIG_REGMAP_MBQ selected. However there's no functional dependency between the two modules so they can be selected separately.
The other thing I'm wondering here is about compatibility - is this something we can enumerate at runtime and if so couldn't this be done more like how we handle the various I2C and SMBus variants so the driver just says it wants a SoundWire regmap and then based on the capabilities of the device and the controller the regmap decides if it can use MBQ or not on the current system?
An SDCA device will have two regmaps, one for 'regular' registers and one for MBQ-based ones. There is no known case where a codec can use ONLY an MBQ-based regmap.
It's different from I2C/SMB since the bus is really identical, the interface is the same, the difference is really the sequence by which you access registers allocated to SDCA and how the address is constructed.
Each SDCA control will be described with a firmware property, and based on their range and purpose you would know how if the control is a regular one or an MBQ-based one. Alternatively, the driver might hard-code things and define addresses for each.
Does this answer to your question?