tests: drivers: uart: async_api: update wba55cg clock frequency

With the current configuration, we encounter a user setting error
during the test with the log:
"Wrong number of bytes received, got: 2, expected: 3."

Workaround:
Increase the clock frequency to enable faster data transmission
and avoid user setting errors.

Signed-off-by: Fabrice DJIATSA <fabrice.djiatsa-ext@st.com>

Author: djiatsaf-st

tests/drivers/uart/uart_async_api/boards/nucleo_wba55cg.overlay

@@ -4,6 +4,11 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+&rcc {
+	clocks = <&clk_hsi>;
+	clock-frequency = <DT_FREQ_M(32)>;
+};
+
 dut: &lpuart1 {
 	dmas = <&gpdma1 0 16 STM32_DMA_PERIPH_TX
 		&gpdma1 1 15 STM32_DMA_PERIPH_RX>;