arch/arm/cpu/arm926ejs/omap/cpuinfo.c: Fix GCC 4.6 warnings

Fix:
cpuinfo.c: In function 'print_cpuinfo':
cpuinfo.c:155:6: warning: variable 'system_serial_low' set but not used
[-Wunused-but-set-variable]
cpuinfo.c:154:6: warning: variable 'system_serial_high' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Anatolij Gustschin <agust@denx.de>

arch/arm/cpu/arm926ejs/omap/cpuinfo.c

@@ -11,6 +11,7 @@
 
 #include <common.h>
 #include <command.h>
+#include <linux/compiler.h>
 
 #if defined(CONFIG_DISPLAY_CPUINFO) && defined(CONFIG_OMAP)
 
@@ -151,8 +152,8 @@ int print_cpuinfo (void)
 	u8 die_rev;
 	u32 omap_id;
 	u8 cpu_type;
-	u32 system_serial_high;
-	u32 system_serial_low;
+	__maybe_unused u32 system_serial_high;
+	__maybe_unused u32 system_serial_low;
 	u32 system_rev = 0;
 
 	jtag_id = omap_get_jtag_id();