MIPS: Loongson64: Select SMP in Kconfig to avoid build error

In the current code, CONFIG_SMP can be set as N by user on the Loongson
platform, then there exists the following build error under !CONFIG_SMP:

  CC      arch/mips/kernel/asm-offsets.s
In file included from ./include/linux/gfp.h:9:0,
                 from ./include/linux/xarray.h:14,
                 from ./include/linux/radix-tree.h:18,
                 from ./include/linux/fs.h:15,
                 from ./include/linux/compat.h:17,
                 from arch/mips/kernel/asm-offsets.c:12:
./include/linux/topology.h: In function 'numa_node_id':
./include/linux/topology.h:119:2: error: implicit declaration of function 'cpu_logical_map' [-Werror=implicit-function-declaration]
  return cpu_to_node(raw_smp_processor_id());
  ^
cc1: some warnings being treated as errors
scripts/Makefile.build:117: recipe for target 'arch/mips/kernel/asm-offsets.s' failed
make[1]: *** [arch/mips/kernel/asm-offsets.s] Error 1

Select SMP in Kconfig to avoid the above build error and then remove
CONFIG_SMP=y in loongson3_defconfig.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>