m68k: use the mc146818rtc.h for non-mmu setups as well.

The mmu varient of mc146818rtc.h can be use on the non-mmu builds as well.
Revert to the single mc146818rtc.h file.

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