STM32 RTC manages only 2 digits for YEAR
(Year tens and units in BCD format in RTC_DR register).
With this patch, RTC driver assumes that tm->tm_years is between
2000 and 2099; tm->tm_year - 2000 have only 2 digit
(0 > and <= 99).
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>