MIPS: Loongson64: Fix build error 'secondary_kexec_args' undeclared under !SMP

On the Loongson64 platform, if CONFIG_SMP is not set, the following build
error will occur:
arch/mips/loongson64/reset.c:133:2: error:'secondary_kexec_args' undeclared

Because the definition and declaration of secondary_kexec_args are in the
CONFIG_SMP, the secondary_kexec_args variable should be used in CONFIG_SMP.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Youling Tang <tangyouling@loongson.cn>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>