m68k: use simpler dma_alloc_coherent() for ColdFire CPUs

On all ColdFire platforms (whether MMU enabled or not) we want to use the
simple page based dma_alloc_coherent. We don't want the virtual mapping
version that is used on classic m68k setups. So modify the conditionals
to use the existing simpler dma_alloc_coherent on all ColdFire and non-MMU
builds.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>