lib/rtc: Add sanity check for time and date

Add a function to check sanity of a given RTC date and time.
Invalid values in terms of overrun ranges of the registers can lead to
strange issues in the OS.

Change-Id: I0a381d445c894eee4f82b50fe86dad22cc587605
Signed-off-by: Werner Zeh <werner.zeh@siemens.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/54913
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>

src/include/rtc.h

@@ -21,5 +21,6 @@ int rtc_get(struct rtc_time *time);
 int rtc_to_tm(int tim, struct rtc_time *tm);
 unsigned long rtc_mktime(const struct rtc_time *tm);
 void rtc_display(const struct rtc_time *tm);
+int rtc_invalid(const struct rtc_time *tm);
 
 #endif /* _RTC_H_ */