Re-integrate UART hardware flow control

[stnolting]

Discussed in #537

^{Originally posted by vhdlnerd March 8, 2023}
I'm disappointed with the recent removal of the UART hardware handshaking lines. This breaks all my neorv32 based designs. I'm a little shocked that functionality would be removed from a basic interface. I use the UARTs at 921600 baud sending commands to the neorv32 that take time to process -- hardware handshake lines are required to avoid dropping commands.

The two workarounds mentioned in the change notes (adding a non-crippled UART as a custom function or a wishbone module) are equally poor. Either way means having a custom bootloader and worst not being able to use any of neov32's SW library functions that uses UARTs (neorv32_uart_printf, ...).

Brian

[vhdlnerd]

Thanks for adding this back in!