MIPS: math-emu: Allow m{f,t}hc emulation on MIPS R6

The mfhc/mthc instructions are supported on MIPS R6 so emulate
them if needed.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: <stable@vger.kernel.org> # 4.0+
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/10737/


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