MIPS: kernel: cps-vec: Set ISA level to mips32r2 for the MIPS MT ASE

Fixes the following build warnings:
arch/mips/kernel/cps-vec.S: Assembler messages:
arch/mips/kernel/cps-vec.S:228: Warning: the `mt' extension requires
MIPS32 revision 2 or greater
[...]
arch/mips/kernel/cps-vec.S: Assembler messages:
arch/mips/kernel/cps-vec.S:345: Warning: the `mt' extension requires
MIPS32 revision 2 or greater

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: Paul Burton <Paul.Burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/7355/


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