m68knommu: modify Makefiles to support common coldfire directory

Modify Makefiles to support separate coldfire platform directory.

Currently the common ColdFire CPU family code sits in the
arch/m68knommu/platform/5307 directory. This is confusing, the files
containing this common code are in no way specific to the 5307 ColdFire.
Create an arch/m68knommu/platform/coldfire directory to contain this
common code. Other m68knommu CPU varients do not need use this code
though, so it doesn't make sense to move it to arch/m68knommu/kernel.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>