Each sequence has to be protected by some lock and this lock needs to be
placed in MFD. Yes the routines for MB access can be for example in DPLL
driver but still the locks have to be inside MFD. So they have to be
exposed to sub-devices.
The point of using MFD was gpio? Does the gpio driver need access to
the mailboxes? Does any other sub driver other than DPLL need to
access mailboxes?
The mutex needs to be shared, but that could be in the common data
structure.