tests: drivers: uart stm32 run uart_async api on f746 nucleo

This sets the dts of dma for using the uart 6 asynch api.
The stm32f746 has a dma V1 with request 5 for Tx/Rx usart6
The Tx&Rx pins (PG14, PG9) of the usart6 are connected
on the nucleo_f746zg board to pass the test
The CONFIG_DCACHE=n must also be set to disable Dcache.

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