ARM: dma: replace ISA_DMA_THRESHOLD with a variable

ISA_DMA_THRESHOLD has been unused by non-arch code, so lets now get
rid of it from ARM by replacing it with arm_dma_zone_mask.  Move
dma_supported() and dma_set_mask() out of line, and have
dma_supported() check this new variable instead.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/mm/mm.h

@@ -23,5 +23,11 @@ extern void __flush_dcache_page(struct address_space *mapping, struct page *page
 
 #endif
 
+#ifdef CONFIG_ZONE_DMA
+extern u32 arm_dma_limit;
+#else
+#define arm_dma_limit ((u32)~0)
+#endif
+
 void __init bootmem_init(void);
 void arm_mm_memblock_reserve(void);