device/dram/ddr3.h: Add brackets around macro

This fixes improper dram frequency being displayed in sandy bridge
native raminit.

Change-Id: I1fe4e4331f45ce1c21113c039b8433252326293d
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/20229
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Sumeet R Pawnikar <sumeet.r.pawnikar@intel.com>

src/include/device/dram/ddr3.h

@@ -52,7 +52,7 @@
  * These values are in 1/256 ns units.
  * @{
  */
-#define NS2MHZ_DIV256 1000 << 8
+#define NS2MHZ_DIV256	(1000 << 8)
 
 #define TCK_1333MHZ     192
 #define TCK_1200MHZ     212