Design better API for adding registers

[whitequark]

Right now all registers are added directly through target and accessed directly via device, which is quite suboptimal as it requires manually tracking addresses and width instead of being able to refer to registers symbolically. We need a better API.

[whitequark]

As a bonus this API should allow third-party code (i.e. not running as a part of Glasgow Python codebase) to discover the register numbers.