target-mips: Clean up microMIPS32 major opcode

  I check MIPS microMIPS manual [1], and found the major opcode might
be wrong. I add a comment to explicitly indicate what manual I am refering
to, and according that manual I remove microMIPS32 major opcodes 0x1f.
As for others, like 0x16, 0x17, 0x36 and 0x37, they are for higher-order
MIPS ISA level or new revision of this microMIPS architecture. Quote
from Johnson, they are belong MIPS64 [2].

[1] http://www.mips.com/products/architectures/micromips/#specifications

    MIPS Architecture for Programmers Volume II-B:
      The microMIPS32 Instruction Set (Revision 3.05)

    MD00582-2B-microMIPS-AFP-03.05.pdf

[2] http://www.mips.com/products/architectures/mips64/

    MIPS Architecture For Programmers
      Volume II-A: The MIPS64 Instruction Set

    MD00087-2B-MIPS64BIS-AFP-03.51.pdf

Signed-off-by: Chen Wei-Ren <chenwj@iis.sinica.edu.tw>
Reviewed-by: Eric Johnson <ericj@mips.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
