samples: boards: stm32: serial_wakeup: Fix nucloe_wb55rg configuration

On STM32WB55 series, wakeup in stop mode is not supported.
Disable this state in order to support this sample.

Add comments to other sections of the configuration.

Signed-off-by: Erwan Gouriou <[email protected]>

Author: erwango

samples/boards/stm32/power_mgmt/serial_wakeup/boards/nucleo_wb55rg.overlay

@@ -1,19 +1,35 @@
 /*
  * Copyright (c) 2022 Linaro Limited
+ * Copyright (c) 2022 STMicroelectronics
  *
  * SPDX-License-Identifier: Apache-2.0
  */
 
-&clk_hsi {
-	status = "okay";
+&cpu0{
+	/* USART Wakeup requires automatic HSI16 switch on in deepsleep mode
+	 * which isn't possible in Stop Mode 2.
+	 * Remove Stop Mode 2 from supported modes
+	 */
+	cpu-power-states = <&stop0 &stop1>;
 };
 
 &usart1 {
+	/* Set domain clock to HSI to allow wakeup from Stop mode */
 	clocks = <&rcc STM32_CLOCK_BUS_APB2 0x00004000>,
 		 <&rcc STM32_SRC_HSI USART1_SEL(2)>;
 
+	/* Configure device as wakeup source */
 	wakeup-source;
 
+	/* Configure sleep pinctrl configuration which will be used when
+	 * device is not configured as wakeup source by the application.
+	 * This use case is only applicable in PM_DEVICE mode.
+	 */
 	pinctrl-1 = <&analog_pb6 &analog_pb7>;
 	pinctrl-names = "default", "sleep";
 };
+
+&clk_hsi {
+	/* Make sure HSI is enabled */
+	status = "okay";
+};