Improve module registers declaration to enable type checking

[kingofpayne]

Currently the registers of classes inheriting from scaffold.Module are populated using the method Module.add_register. This method automatically adds attributes to the modules with the register name (prefixed with "reg_").

Python code analysers, for example type inference plugins or autocompletion, fail to understand that.
Instead of writing:

self.add_register("status", "rw", address)

We could write :

self.reg_status = add_register("rw", address)