rtc: add a timestamp for year 0

A few RTCs handle dates from year 0 to year 9999. Add a timestamp even if
years before 1970 will probably never be used.

Link: https://lore.kernel.org/r/20191007134724.15505-1-alexandre.belloni@bootlin.comSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

include/linux/rtc.h

@@ -160,6 +160,7 @@ struct rtc_device {
 #define to_rtc_device(d) container_of(d, struct rtc_device, dev)
 
 /* useful timestamps */
+#define RTC_TIMESTAMP_BEGIN_0000	-62167219200ULL /* 0000-01-01 00:00:00 */
 #define RTC_TIMESTAMP_BEGIN_1900	-2208988800LL /* 1900-01-01 00:00:00 */
 #define RTC_TIMESTAMP_BEGIN_2000	946684800LL /* 2000-01-01 00:00:00 */
 #define RTC_TIMESTAMP_END_2063		2966371199LL /* 2063-12-31 23:59:59 */