m68k: merge mmu and non-mmu versions of time.c

There is only trivial differences between the mmu time_mm.c and non-mmu
time_no.c files. Merge them back into a single time.c.

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