drivers: serial: stm32 uart async is valid for stm32U5 serie
drivers: serial: stm32 uart async is valid for stm32U5 serie

The UART_ASYNC is supported on the stm32U5 serie through the
GPDMA driver.

Signed-off-by: Francois Ramu <b83adc7f8a23ee911a37fcb61c4033581aae0b7a@st.com>

zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr