Add Rocket UART example

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Rocket-chip is a famous SoC generator with a rich number of features.

This PR adds a guide that shows the possibility of running rocket on Tang Primer 20K with adequate config and some tweaks.

This guide also serves as an example FPGA flow with open source tools yosys and openFPGALoader and gowin command line tool gw_sh.

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It is reported that UART does not work with the RV debugger. I used a FT2232 to get the UART output (baudrate 115200).

Credit to @Icenowy for this. I've done the tweaks months ago but can not get the UART working until today.