ARM: mm: fix type of the arm_dma_limit global variable

arm_dma_limit stores physical address of maximal address accessible by DMA,
so the phys_addr_t type makes much more sense for it instead of u32. This
patch fixes the following build warning:

arch/arm/mm/init.c:380: warning: comparison of distinct pointer types lacks a cast
Reported-by: NRussell King <linux@arm.linux.org.uk>
Signed-off-by: NMarek Szyprowski <m.szyprowski@samsung.com>

arch/arm/mm/init.c

@@ -212,7 +212,7 @@ EXPORT_SYMBOL(arm_dma_zone_size);
  * allocations.  This must be the smallest DMA mask in the system,
  * so a successful GFP_DMA allocation will always satisfy this.
  */
-u32 arm_dma_limit;
+phys_addr_t arm_dma_limit;
 
 static void __init arm_adjust_dma_zone(unsigned long *size, unsigned long *hole,
 	unsigned long dma_size)

arch/arm/mm/mm.h

@@ -62,7 +62,7 @@ extern void __flush_dcache_page(struct address_space *mapping, struct page *page
 #endif
 
 #ifdef CONFIG_ZONE_DMA
-extern u32 arm_dma_limit;
+extern phys_addr_t arm_dma_limit;
 #else
 #define arm_dma_limit ((u32)~0)
 #endif