fix(pm): Restore sleep suspension of devices.

* After the move to `sys_poweroff`, restore the behavior of
  suspending devices before entering sleep state.

app/include/linker/zmk-pm-devices.ld

@@ -0,0 +1,9 @@
+/*
+ * Copyright (c) 2023 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <zephyr/linker/linker-defs.h>
+
+ITERABLE_SECTION_RAM(zmk_pm_device_slots, 4)