[MIPS] Qemu: Fix Symmetric Uniprocessor support.

Might be useful for SMP debugging.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

[Rewritten Kconfig bits to deal better fit in the usual pattern of doing
things - Ralf]

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>