m68k: Implement arch_dma_prep_coherent()

When we remap memory as non-cached, to be used as a DMA coherent buffer,
we should writeback all cache and invalidate the cache lines so that we
make sure we have a clean slate.  Implement this using the cache_push()
helper.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/Kconfig

@@ -4,6 +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_SYNC_DMA_FOR_DEVICE if HAS_DMA
 	select ARCH_MIGHT_HAVE_PC_PARPORT if ISA
 	select ARCH_NO_COHERENT_DMA_MMAP if !MMU

arch/m68k/kernel/dma.c

@@ -18,6 +18,11 @@
 #include <asm/pgalloc.h>
 
 #if defined(CONFIG_MMU) && !defined(CONFIG_COLDFIRE)
+void arch_dma_prep_coherent(struct page *page, size_t size)
+{
+	cache_push(page_to_phys(page), size);
+}
+
 pgprot_t arch_dma_mmap_pgprot(struct device *dev, pgprot_t prot,
 		unsigned long attrs)
 {