dma-mapping: remove arch_dma_mmap_pgprot

arch_dma_mmap_pgprot is used for two things:

 1) to override the "normal" uncached page attributes for mapping
    memory coherent to devices that can't snoop the CPU caches
 2) to provide the special DMA_ATTR_WRITE_COMBINE semantics on older
    arm systems and some mips platforms

Replace one with the pgprot_dmacoherent macro that is already provided
by arm and much simpler to use, and lift the DMA_ATTR_WRITE_COMBINE
handling to common code with an explicit arch opt-in.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>	# m68k
Acked-by: Paul Burton <paul.burton@mips.com>		# mips

arch/m68k/kernel/dma.c

@@ -23,8 +23,7 @@ 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)
+pgprot_t pgprot_dmacoherent(pgprot_t prot)
 {
 	if (CPU_IS_040_OR_060) {
 		pgprot_val(prot) &= ~_PAGE_CACHE040;