MIPS: Prevent building MT support for microMIPS kernels

We don't currently support the MT ASE for microMIPS kernels, and there
are no CPUs currently in existence that use both. They can however both
be enabled in Kconfig, resulting in build failures such as:

  AS      arch/mips/kernel/cps-vec.o
arch/mips/kernel/cps-vec.S: Assembler messages:
arch/mips/kernel/cps-vec.S:242: Warning: the 32-bit microMIPS architecture does not support the `mt' extension
arch/mips/kernel/cps-vec.S:276: Error: unrecognized opcode `mttc0 $13,$2,2'
arch/mips/kernel/cps-vec.S:282: Error: unrecognized opcode `mttc0 $8,$1,2'
arch/mips/kernel/cps-vec.S:285: Error: unrecognized opcode `mttc0 $0,$2,1'
...

Fix this by preventing MT from being enabled when targeting microMIPS.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16951/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>