rtc-m48t59: shift zero year to 1968 on sparc (rev 2)

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: Krzysztof Helt <krzysztof.h1@wp.pl>
Tested-by: Alexander Beregalov
Signed-off-by: David S. Miller <davem@davemloft.net>