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>

arch/m68k/Kconfig

@@ -4,7 +4,7 @@ config M68K
 	default y
 	select ARCH_32BIT_OFF_T
 	select ARCH_HAS_DMA_MMAP_PGPROT if MMU && !COLDFIRE
-	select ARCH_HAS_DMA_PREP_COHERENT
+	select ARCH_HAS_DMA_PREP_COHERENT if HAS_DMA && MMU && !COLDFIRE
 	select ARCH_HAS_SYNC_DMA_FOR_DEVICE if HAS_DMA
 	select ARCH_MIGHT_HAVE_PC_PARPORT if ISA
 	select ARCH_NO_COHERENT_DMA_MMAP if !MMU