Refine the busdma template interface. Provide tools for filling in fields

Refine the busdma template interface.  Provide tools for filling in fields
that can be extended, but also ensure compile-time type checking.  Refactor
common code out of arch-specific implementations.  Move the mpr and mps
drivers to this new API.  The template type remains visible to the consumer
so that it can be allocated on the stack, but should be considered opaque.