STM32 USB device IP port

[benedekkupper]

I do want to port this project to STM32. I briefly considered using the HAL libraries this time, but after a short time I realized it's (still) very cumbersome to cleanly integrate anything into the HAL project design. So my plan is to refactor the old register-level port of the USB device IP of STM32 MCUs with simpler integration into STM32 projects. This will allow using proper endpoint buffer allocations, which is a key for getting the flexibility needed for composite designs.

The same would need to be done for the STM32 / Synopsis OTG IP, however the complexity of that driver is just so much higher that I don't want to deal with it at this point.