m68knommu: make persistent clock code consistent with m68k

The read_persistent_clock() code is different on m68knommu, for really no
reason. With a few changes to support function names and some code
re-organization the code can be made the same.

This will make it easier to merge the arch/m68k/kernel/time.c for m68k and
m68knommu in a future patch.

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