Provide read-only access via address alias or similar to allow fpga register dumping

[nathanaelhuffman]

Some registers have read side effects, we'd like to not accidentally trip those when doing a dump.

[nathanaelhuffman]

Ideally what we'd get here is some side-band decoding in our non-existant fabric generator that would strip off msb, and pass no_read_side-effects or something into a block maybe?