m68k: use mmu timex.h for non-mmu setups as well

The non-mmu timex.h can be cleaned up and ends up being identical
to the mmu timex.h, just just use that.

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