Add a ROT13 workshop example

[mibus]

This is similar to the RGB litex/workshop example, but wires two CSRs together for a slightly more complicated example.

Poke a byte value into the "input" CSR via wishbone-tool, and retrieve the ROT13-encoded output byte from the "output" CSR.

[mithro]

FYI - @tcal-x

[mithro]

Next step is to do that with a FIFO on each end :-)

[mibus]

Next step is to do that with a FIFO on each end :-)

If you think I should, I'm happy to explore that and submit a PR for that instead. This was just me exploring wiring up signals, and (so far) CSRs are all I've learned!