tests: drivers: uart async with DMA on stm32fx nucleo144 boards

It enables the usart6 to run the testcase with a DMA
tests/drivers/uart/uart_async_api. DMA2 (of type V1)
is configured on channel 5 (request) streams 7 & 2.
USART Tx and Rx PG14 - PG9 pins (14 & 16 of CN10)
are connected to PASS the test.

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