m68knommu: cleanup m68knommu timer code

Reduce the function pointer mess of the m68knommu timer code by calling
directly to the local hardware's timer setup, and expose the local
common timer interrupt handler to the lower level hardware timer.

Ultimately this will save definitions of all these functions across all
the platform code to setup the function pointers (which for any given
m68knommu CPU family member can be only one set of hardware timer
functions).

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>