m68knommu: modify timer init code to make it consistent with m68k code

With a few small changes we can make the m68knommu timer init code the
same as the m68k code. By using the mach_sched_init function pointer
and reworking the current timer initializers to keep track of the common
m68k timer_interrupt() handler we end up with almost identical code for
m68knommu.

This will allow us to more easily merge the mmu and non-mmu m68k time.c
in future patches.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>