refactor: Move to new sys_poweroff API.

* Move to new `sys_poweroff` API for our deep sleep functionality.

app/Kconfig

@@ -387,13 +387,12 @@ config ZMK_IDLE_TIMEOUT
 
 config ZMK_SLEEP
     bool "Enable deep sleep support"
+    depends on HAS_POWEROFF
+    select POWEROFF
     imply USB
 
 if ZMK_SLEEP
 
-config PM_DEVICE
-    default y
-
 config ZMK_IDLE_SLEEP_TIMEOUT
     int "Milliseconds of inactivity before entering deep sleep"
     default 900000

app/src/activity.c

@@ -7,7 +7,7 @@
 #include <zephyr/device.h>
 #include <zephyr/init.h>
 #include <zephyr/kernel.h>
-#include <zephyr/pm/pm.h>
+#include <zephyr/sys/poweroff.h>
 
 #include <zephyr/logging/log.h>
 
@@ -70,7 +70,7 @@ void activity_work_handler(struct k_work *work) {
     if (inactive_time > MAX_SLEEP_MS && !is_usb_power_present()) {
         // Put devices in suspend power mode before sleeping
         set_state(ZMK_ACTIVITY_SLEEP);
-        pm_state_force(0U, &(struct pm_state_info){PM_STATE_SOFT_OFF, 0, 0});
+        sys_poweroff();
     } else
 #endif /* IS_ENABLED(CONFIG_ZMK_SLEEP) */
         if (inactive_time > MAX_IDLE_MS) {