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: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
index c21d06c..f54d592 100644
--- a/arch/arm/mm/init.c
+++ b/arch/arm/mm/init.c
@@ -212,7 +212,7 @@
  * 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)