tests: drivers: uart: elementary: stm32: add support for hw flow control

- Configure and attach DMA for each USART node.
- Enable hardware flow control capability.
- Add RTS/CTS pin name signals.

Note: Some platform series are missing due to conflicts with other pins
used by different test drivers
(RTS pin should be connected to the CTS pin.)

Signed-off-by: Fabrice DJIATSA <fabrice.djiatsa-ext@st.com>