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.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>