Clarify interface for get_register_address

[vtangTT]

As per comments from @joelsmithTT on #40

I think the 'register' part of get_register_address is a bit misleading here.. if I understand the code correctly, it can end up writing to a register exposed in a PCIe BAR, or write through an inbound TLB window (to what might be either memory or a register).

It is probably worth making a distinction between these scenarios at the interface level.