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/mm.h b/arch/arm/mm/mm.h
index 93dc0c1..c471436 100644
--- a/arch/arm/mm/mm.h
+++ b/arch/arm/mm/mm.h
@@ -62,7 +62,7 @@
 #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