m68k: move non-mmu 68000 platform code

The non-mmu 68000 specific code is inconsistently placed under a directory
named "platform". Move it to arch/m68k/ along with the other platform and
machine directories.

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