soc: st: stm32: Add poweroff to F4 family

Allows F4 MCUs to enter standby mode which behave similar to the poweroff
mode.

Signed-off-by: Tomáš Juřena <jurenatomas@gmail.com>

soc/st/stm32/stm32f4x/CMakeLists.txt

@@ -12,3 +12,4 @@ set(SOC_LINKER_SCRIPT ${ZEPHYR_BASE}/include/zephyr/arch/arm/cortex_m/scripts/li
 zephyr_sources_ifdef(CONFIG_PM
   power.c
   )
+zephyr_sources_ifdef(CONFIG_POWEROFF poweroff.c)

soc/st/stm32/stm32f4x/Kconfig

@@ -12,4 +12,5 @@ config SOC_SERIES_STM32F4X
 	select CPU_HAS_ARM_MPU
 	select HAS_SWO
 	select HAS_PM
+	select HAS_POWEROFF
 	select SOC_EARLY_INIT_HOOK

soc/st/stm32/stm32f4x/poweroff.c

@@ -0,0 +1,29 @@
+/*
+ * Copyright (c) 2023 Nordic Semiconductor ASA
+ * Copyright (c) 2024 STMicroelectronics
+ * Copyright (c) 2025 Tomas Jurena
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr/kernel.h>
+#include <zephyr/sys/poweroff.h>
+#include <zephyr/toolchain.h>
+#include <zephyr/drivers/misc/stm32_wkup_pins/stm32_wkup_pins.h>
+
+#include <stm32_ll_cortex.h>
+#include <stm32_ll_pwr.h>
+
+void z_sys_poweroff(void)
+{
+#ifdef CONFIG_STM32_WKUP_PINS
+	LL_PWR_ClearFlag_WU();
+#endif /* CONFIG_STM32_WKUP_PINS */
+
+	LL_PWR_SetPowerMode(LL_PWR_MODE_STANDBY);
+	LL_LPM_EnableDeepSleep();
+
+	k_cpu_idle();
+
+	CODE_UNREACHABLE;
+}