tests: drivers: uart async on stm32u585 disco kit

This commit configures the testcase to run on the
stm32u585 target boards. USART3 is selected
Pins Tx and Rx (PD8 PD9 on CN14) must be connected on the HW board
to pass the test.
The USART transfer uses the GP DMA transfer with request
28 and 29 on 2 DMA channels (0-15).

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