stm32f0: rtc: set up the real-time clock and implement functions to get and set the time

include/rtc.h

@@ -0,0 +1,7 @@
+#pragma once
+
+#include <time.h>
+
+int rtc_init(struct tm *time);
+int rtc_get_time(struct tm *time);
+time_t rtc_time(void);