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>