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: NChen Wei-Ren <chenwj@iis.sinica.edu.tw>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>
(cherry picked from commit 6801038b)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

target-mips/translate.c

@@ -9486,7 +9486,7 @@ enum {
 
     /* bits 13..12 for 0x32 */
     MULT_ACC = 0x0,
-    MULTU_ACC = 0x0,
+    MULTU_ACC = 0x1,
 
     /* bits 15..12 for 0x2c */
     SEB = 0x2,