[bsp]add stm32f4xx bsp rtc driver

bsp/stm32f40x/drivers/stm32f4_rtc.h

@@ -0,0 +1,23 @@
+/*
+ * File      : stm32f4_rtc.h
+ * This file is stm32f4 rtc driver part of RT-Thread RTOS
+ * COPYRIGHT (C) 2009, RT-Thread Development Team
+ *
+ * The license and distribution terms for this file may be
+ * found in the file LICENSE in this distribution or at
+ * http://www.rt-thread.org/license/LICENSE
+ *
+ * Change Logs:
+ * Date           Author       Notes
+ * 2015-09-29     FlyM	       the first version
+ */
+
+#ifndef __STM32F4_RTC_H__
+#define __STM32F4_RTC_H__
+
+#define RTC_CLOCK_SOURCE_LSE
+
+void rt_hw_rtc_init(void);
+
+#endif
+