mn10300: make dma_cache_sync a no-op

mn10300 does not implement DMA_ATTR_NON_CONSISTENT allocations, so it
doesn't make any sense to do any work in dma_cache_sync given that it must
be a no-op when dma_alloc_attrs returns coherent memory.

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