target-mips: fix decoding of microMIPS POOL32Axf instructions

Fix incorrect assumption that DSP and non-DSP versions of the following
instructions have the same encoding:
MULT, MULTU, MADD, MADDU, MSUB, MSUBU, MFHI, MFLO, MTHI, MTLO.
Correct the existing (non-DSP) instructions and add DSP equivalents.

Reference:
MIPS Architecture for Programmers Volume II-B: The microMIPS32
Instruction Set
MIPS Architecture for Programmers Volume IV-e: The MIPS DSP Module for
the microMIPS32 Architecture

Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>