MIPS: Skip IPI setup if we only have 1 CPU

If we're running on a system with only 1 possible CPU then it makes no
sense to reserve or initialise IPIs since we'll never use them. Avoid
doing so.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16192/


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