MIPS: MT: core_nvpes function to retrieve VPE count

This function simply returns the number of VPEs present in the current
core, or 1 if the core does not implement the MT ASE. In SMP kernels
this will typically equal smp_num_siblings, however it will also be
usable in UP kernels and helps prepare for the possibility of a
heterogenous system where the VPE count is not the same across all
cores.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6665/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>