Shift the first year to 1968 for Sun SPARC machines.

Move this logic from platform specific files to rtc driver
as this fixes problems with calculating a century bit.
Signed-off-by: NKrzysztof Helt <krzysztof.h1@wp.pl>
Tested-by: Alexander Beregalov
Signed-off-by: NDavid S. Miller <davem@davemloft.net>