Fix mips_cm_max_vp_width for UP kernels where it previously referenced
smp_num_siblings, which is not declared for UP kernels. This led to
build errors such as the following:

  drivers/built-in.o: In function `$L446':
  irq-mips-gic.c:(.text+0x1994): undefined reference to `smp_num_siblings'
  drivers/built-in.o:irq-mips-gic.c:(.text+0x199c): more undefined references to `smp_num_siblings' follow

On UP kernels simply return 1, leaving the reference to smp_num_siblings
in place only for SMP kernels.
Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Matt Redfearn <matt.redfearn@imgtec.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12332/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>