cores: add simple 8-bit gpio core

Cleaned up version of the old orpsocv2 gpio core.
The "configurable" width have been removed, since the
layout of data and dir registers didn't make any sense.
If wider width is desired, it's expected to instantiate
several verisons of the core instead.
The tristate logic have been moved outside of the core,
and an output enable signal has been added instead.
The old core only checked for wb_stb_i and not wb_cyc_i,
that has been fixed.