Experiment with a hardware driver for a 2MHz 6522 (Master only)

[hoglet67]

Suggestion from Steve Picton:

I was tinkering with MMFST on the '2MHz Econet' in the Master and was surprised that it wasn't much quicker than the standard User Port. It seems that there are quite a lot of instructions in each loop and it stays on around 21us per byte. I did patch it to get 10.5us/byte but it might be worth introducing a variant which is specific to the 6522 on the Econet port.