target-mips: fix wrong microMIPS opcode encoding

target-mips: fix wrong microMIPS opcode encoding

While reading microMIPS decoding, I found a possible wrong opcode
encoding. According to [1] page 166, the bits 13..12 for MULTU is
0x01 rather than 0x00. Please review, thanks.

[1] MIPS Architecture for Programmers VolumeIV-e: The MIPS DSP
    Application-Specific Extension to the microMIPS32 Architecture

Signed-off-by: Chen Wei-Ren <chenwj@iis.sinica.edu.tw>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
(cherry picked from commit 6801038bc52d61f81ac8a25fbe392f1bad982887)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>