dma-mapping: add a Kconfig symbol to indicate arch_dma_prep_coherent presence

Add a Kconfig symbol that indicates an architecture provides a
arch_dma_prep_coherent implementation, and provide a stub otherwise.

This will allow the generic dma-iommu code to use it while still
allowing to be built for cache coherent architectures.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>

arch/csky/Kconfig

@@ -1,6 +1,7 @@
 config CSKY
 	def_bool y
 	select ARCH_32BIT_OFF_T
+	select ARCH_HAS_DMA_PREP_COHERENT
 	select ARCH_HAS_SYNC_DMA_FOR_CPU
 	select ARCH_HAS_SYNC_DMA_FOR_DEVICE
 	select ARCH_USE_BUILTIN_BSWAP