Add factory registration for top-level blocks

[mballance]

This PR adds factory registration for register blocks.
UVM encourages registering component and object classes with the factory. While it's not necessary to register all classes (eg individual registers), registering top-level classes is a good compromise.
In my case, the UVM tool I use to create environments (UVM Framework) assumes that the top-level register block can be created via the factory.

[umarcis]

I was also facing the same issue, and have to add this particular line of factory registration in register model myself, then it runs without any error, however it should be approved so that everyone use it without facing this issue...