MIPS: smp-cps: Add support for CPU hotplug of MIPSr6 processors

MIPS: smp-cps: Add support for CPU hotplug of MIPSr6 processors

Introduce support for hotplug of Virtual Processors in MIPSr6 systems.
The method is simpler than the VPE parallel from the now-deprecated MT
ASE, it can now simply write the VP_STOP register with the mask of VPs
to halt, and use the VP_RUNNING register to determine when the VP has
halted.

Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Reviewed-by: Paul Burton <paul.burton@imgtec.com>
Cc: Matt Redfearn <matt.redfearn@imgtec.com>
Cc: Qais Yousef <qais.yousef@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/13752/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>