UARTResource gets a new argument, "role", which determines flow
control signal directions:
- DCE means that the design acts as a modem
- DTE means that the design acts as a PC

Pin numbers match the P0/P1 signals in the schematic, but the
direction is reversed. Fix by setting role="dce".

Ref. https://github.com/mystorm-org/BlackIce-II/blob/master/hardware/BlackIce.pdf

According to the schematic, RTS and CTS are not connected to CH340G in
this version of the board.

Ref. https://github.com/monsonite/BackIce_FPGA/blob/master/BlackIce18_07_01D.pdf
and https://forum.mystorm.uk/uploads/default/original/1X/a5db1ce1c9bc2d91e63cfdc8424d699c2419a3d0.png

According to the schematic, RTS is E5 and CTS is D3. Previously these
were reversed to work around signal direction set in UARTResource.
Un-reverse the signals, and set correct direction by passing role=dce.

Ref. https://reference.digilentinc.com/_media/nexys4-ddr:nexys_4_ddr_sch.pdf

RTS and CTS match the schematic, but the direction is incorrect:
CTS is output, RTS is input. Fix by setting role=dce.

Ref. https://www.intel.com/content/dam/altera-www/global/en_US/portal/dsn/42/doc-us-dsnbk-42-5804152209-de0-user-manual.pdf

RTS/CTS and DTR/DSR pairs have been swapped to work around the signal
direction in UARTResource. Un-reverse the signals, making the names
match the schematic. Fix the direction by setting role=dce.

Ref. http://www.latticesemi.com/view_document?document_id=50373

UART flow control pins match the signal names in the schematic, but
directions are reversed. Fix by setting role=dce.