sparc: convert to arch_gettimeoffset()

This patch converts sparc (specifically sparc32) to use GENERIC_TIME via
the arch_getoffset() infrastructure, reducing the amount of arch
specific code we need to maintain.

The sparc architecture is one of the last 3 arches that need to be
converted.

This patch applies on top of Linus' current -git tree

I've taken my best swing at converting this, but I'm not 100% confident
I got it right. My cross-compiler is now out of date (gcc4.2) so I
wasn't able to check if it compiled. Any assistance from arch
maintainers or testers to get this merged would be great.

Signed-off-by: John Stultz <johnstul@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>