LWMON5: POST RTC fix

Modify the RTC API to provide one a status for the time reported by
the rtc_get() function:
  0 - a reliable time is guaranteed,
< 0 - a reliable time isn't guaranteed (power fault, clock issues,
      and so on).

The RTC chip drivers are responsible for providing this info if the
corresponding chip supports such functionality. If not - always
report that the time is reliable.

The POST RTC test was modified to detect the RTC faults utilizing
this new rtc_get() feature.

Signed-off-by: Yuri Tikhonov <yur@emcraft.com>
diff --git a/include/rtc.h b/include/rtc.h
index 15f3571..2995144 100644
--- a/include/rtc.h
+++ b/include/rtc.h
@@ -52,7 +52,7 @@
 	int tm_isdst;
 };
 
-void rtc_get (struct rtc_time *);
+int rtc_get (struct rtc_time *);
 void rtc_set (struct rtc_time *);
 void rtc_reset (void);