sh: Finish the sh64 migration off of ARCH_USES_GETTIMEOFFSET.

This adds sh_tmu support to the SH-5 subtypes, which subsequently allows
us to kill off time_64.c and use the now generic time_32.c. As a bonus,
SH-5 now supports highres timers and tickless for the first time.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>