MIPS: only register MT SMP ops if MT is supported

Verify that the current CPU actually supports multi-threading before
registering MT SMP ops, instead of unconditionally registering them if
the kernel is compiled with CONFIG_MIPS_MT_SMP.

Suggested-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Sander Vanheule <sander@svanheule.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>