MIPS: Kconfig: Make MIPS_MT_SMP a regular Kconfig symbol

Following the removal of SMTC, MIPS_MT_SMP is the only available
MT/SMP option so make it a regular Kconfig symbol.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: Markos Chandras <markos.chandras@imgtec.com>
Patchwork: https://patchwork.linux-mips.org/patch/6720/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>