Is your enhancement proposal related to a problem? Please describe.
On the stm32f411 errata sheet, at 2.5.5, there is a hardware flow control problem
described (nRTS signal abnormally driven low after a protocol violation).

Describe the solution you'd like
I would like to add to the USART bindings an optional property for the rts pin.
The reason is that there is no way of knowing from the pinctrl-0 list, which pin is rts, because
the pinmux only contains: port, pin, and alternative function.
Then if hw-flow-control is enabled, only use CTS (if it is the CONFIG_SOC_STM32F411XE), and configure the
rts pin, and in async mode, when changing buffers set the pin high and lower it when finished.
The same when rx_disable occurs, and set it low again when enabled.

Describe alternatives you've considered
We could also instead of adding the rx_pin in the usart bindings, add pinctrl_names, as with the dma.

Additional context
Found out about this issue when testing the current async implementation, using CONFIG_UART_ASYNC_API
and running it on an stm32f411e_disco with hw-flow-control.
Had issues running it with flow control, and then found about the issue in the errata.