driver/ddr: Fix DDR4 driver for ARM

Previously the driver was only tested on Power SoCs. Different barrier
instructions are needed for ARM SoCs.

Signed-off-by: York Sun <yorksun@freescale.com>
diff --git a/arch/arm/include/asm/io.h b/arch/arm/include/asm/io.h
index 214f3ea..6d18eb3 100644
--- a/arch/arm/include/asm/io.h
+++ b/arch/arm/include/asm/io.h
@@ -136,6 +136,7 @@
  * TODO: The kernel offers some more advanced versions of barriers, it might
  * have some advantages to use them instead of the simple one here.
  */
+#define mb()		asm volatile("dsb sy" : : : "memory")
 #define dmb()		__asm__ __volatile__ ("" : : : "memory")
 #define __iormb()	dmb()
 #define __iowmb()	dmb()