MIPS: Fix SMP core calculations when using MT support.

The TCBIND register is only available if the core has MT support. It
should not be read otherwise. Secondly, the number of TCs (siblings)
are calculated differently depending on if the kernel is configured
as SMVP or SMTC.

Signed-off-by: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
Signed-off-by: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5822/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>