m68knommu: fix non-mmu classic 68000 legacy timer tick selection

The family of classic 68000 parts supported when in non-mmu mode all
currently use the legacy timer support. Move the selection of that config
option, LEGACY_TIMER_TICK, into the core CPU configuration.

This fixes compilation if no specific CPU variant is selected, since
the LEGACY_TIMER_TICK option was only selected in the specific CPU
variant configurations.

Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>