microMIPS: Add MT ASE instruction set support

Add the MT ASE instruction operand types and encodings to the microMIPS
opcode table and enable the assembly of these instructions in GAS from
MIPSr2 onwards.  Update the binutils and GAS testsuites accordingly.

References:

"MIPS Architecture for Programmers, Volume IV-f: The MIPS MT Module for
the microMIPS32 Architecture", MIPS Technologies, Inc., Document Number:
MD00768, Revision 1.12, July 16, 2013

Co-Authored-By: Maciej W. Rozycki <macro@redhat.com>