devantech-eth008: declare more models, still digital-output only

Add the ETH002, ETH484, and ETH8020 items to the list of supported
models. Adjust driver code paths for the differing DO image sizes of
these models (up to 20 relays which span up to three bytes width).
Support the R5..R8 gap for ETH484.

This was written without access to the hardware, and is untested.