power: device: deduplicate suspending code

Function to put devices in lower power mode were all implemented in the
same way. Deduplicate code there by implementing single function to
handle all cases.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>

include/device.h

@@ -361,6 +361,13 @@ __syscall struct device *device_get_binding(const char *name);
 
 #endif /* CONFIG_DEVICE_POWER_MANAGEMENT */
 
+/**
+ * @brief Get name of device PM state
+ *
+ * @param state State id which name should be returned
+ */
+const char *device_pm_state_str(u32_t state);
+
 /**
  * @brief Indicate that the device is in the middle of a transaction
  *