[Cryptech Core] nit: reset low or high

[Paul Selkirk]

This is mostly for Joachim and Pavel, but spammed to the list so we have
a record of design decisions.

Joachim's cores have an active-low reset line, while Pavel (and Bunnie)
have active-high reset. AFAICT Joachim gets an active-low reset from the
Terasic board, and just propagates it to the other cores in his design.
Where I'm mixing cores from different authors, I have to remember which
ones are low and which are high. So e.g. in the Terasic design, we have
reset_n port at the top-level, which I invert to sys_rst for
core_selector. In hash_selector, this gets re-inverted back to reset_n
for the sha cores.

Is there a technical reason to prefer active-low, or should I change all
derived/propagated resets to be active-high for consistency?

				paul