MIPS: smp-mt: Fix link error when PROC_FS=n

MIPS: smp-mt: Fix link error when PROC_FS=n

Commit d6d3c9afaab4 (MIPS: MT: proc: Add support for printing VPE and TC
ids) causes a link error when CONFIG_PROC_FS=n:

arch/mips/built-in.o: In function `proc_cpuinfo_notifier_init':
smp-mt.c: undefined reference to `register_proc_cpuinfo_notifier'

This is fixed by adding an ifdef around the procfs handling code
in smp-mt.c.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Reported-by: Markos Chandras <markos.chandras@imgtec.com>
Reviewed-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: <stable@vger.kernel.org> # >= 3.15
Patchwork: https://patchwork.linux-mips.org/patch/7244/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>