tests: drivers: uart stm32 run uart_async api on l475 disco

This sets the dts of dma for using the uart asynch api.
The stm32l475 has a dmamux with request 2 for Tx/Rx usart4
The Tx&Rx pins (PA0, PA1) of the usart4 are connected
on the disco_l475_iot1 board to pass the test.


Signed-off-by: Francois Ramu <francois.ramu@st.com>