m68k: remove the unused arch/m68k/platform directory

The non-mmu platform/machine directories have been move up one level.
We no longer need arch/m68k/platform, or its Makefile, so remove them.

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