tests: drivers: uart async api test case on stm32u575 nucleo board

This commit configures the testcase to run on the
stm32u5x5 target boards. USART2 is selected
Pins Tx and Rx (PD5 PD6 on CN9) must be connected on the HW board
to pass the test.
The USART transfer uses the GP DMA transfer with request
27 and 26 on 2 DMA channels (0-15).

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