m68k: Don't select ARCH_HAS_DMA_PREP_COHERENT for nommu or coldfire

M68k only provides the arch_dma_prep_coherent symbol when an mmu is
enabled and not on the coldfire platform.  Fix the Kconfig symbol
selection up to match this.

Fixes: 69878ef4

 ("m68k: Implement arch_dma_prep_coherent()")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>