MIPS: CM: Fix compilation error when !MIPS_CM

Fix mips_cm_lock_other compilation error when MIPS_CM is not selected.
This was introduced in commit 23d5de8e

 (MIPS: CM: Introduce core-other
locking functions)

Signed-off-by: Tony Wu <tung7970@gmail.com>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/11698/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>