Add System Verilog FPU (#163)

* Change reset strategy in ariane_verilog_wrap.sv, remove unneeded sigs in serpent_peripherals.

* saving...

*  Updates for new FPU

* Add sv fpu to FPGA flow

* Use multi-threading capabilities of verilator

- Deactivate non-standard floating point arguments
- Make multi-threading conditional on the availability of verilator 4

* Remove DPI threadsafety

* Reduce FPGA clock frequency

- Remove couple of -v- tests to reduce test-time

* Fix documentation and fpga flow

- Fix cycle time to accommodate FPU
- Fix FPGA constraints

* Change UART frequency