MIPS: math-emu: Add support for the MIPS R6 MSUBF FPU instruction

MIPS: math-emu: Add support for the MIPS R6 MSUBF FPU instruction

MIPS R6 introduced the following instruction:
Floating Point Fused Multiply Subtract:
MSUBF.fmt To perform a fused multiply-subtract of FP values.

MSUBF.fmt: FPR[fd] = FPR[fd] - (FPR[fs] x FPR[ft])

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/10957/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>