m68k: don't provide arch_dma_alloc for nommu/coldfire

Coldfire cores configured with a data cache can't provide coherent
DMA allocations at all.

Instead of returning non-coherent kernel memory in this case,
return NULL and fail the allocation.

The only driver that used to rely on the previous behavior (fec) has
been switched to use non-coherent allocations for this case recently.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Greg Ungerer <gerg@linux-m68k.org>
Tested-by: Greg Ungerer <gerg@linux-m68k.org>