Port openvizsla to artix 7

I'm opening this issue to track my slow but sure effort to port openvizsla to the arty artix-7 board.

I've done a pmod for the USB front end (https://github.com/TomKeddie/prj-pmod/tree/master/hardware/pmod-usb-analyser-openvizla/qfn) and am in the process of verifying the hardware. Once verified this would let openvizsla run on other hardware more easily.

I've wired the differential pair to match the pair allocation on the arty-7 pmods but it seems these are not used by the hdl at the moment anyway.

The arty seems like a good target

it's available and not super expensive (they even come up s/h from time to time)
it is universal hardware so it can be used for more than usb analysis
it has dram (suitability tbd)
it has an ft2232h onboard (schematic page is hidden, suitability tbd)
it has ethernet (ideally I'd like to move to an ethernet transport and remove the dependence of the ft2232h).

openvizsla / ov_ftdi