MIPS/opcodes: Free up redundant `g' operand code

In the operand handling rewrite made for the MIPS disassembler with
commit ab902481 ("Add structures to describe MIPS operands"),
<https://sourceware.org/ml/binutils/2013-07/msg00135.html>, the `g'
operand code has become redundant for the regular MIPS instruction set
by duplicating the OP_REG_COPRO semantics of the `G' operand code.

Later commit 351cdf24 ("Implement O32 FPXX, FP64 and FP64A ABI
extensions") converted the CTTC1 instruction from the `g' to the `G'
operand code, but still left a few instructions behind.

Convert the three remaining instructions still using the `g' code then,
namely: CTTC2, MTTC2 and MTTHC2, and remove all traces of the operand
code, freeing it up for other use.

	opcodes/
	* mips-opc.c (mips_builtin_opcodes): Switch "cttc2", "mttc2",
	and "mtthc2" to using the `G' rather than `g' operand code for
	the coprocessor control register referred.

	include/
	* opcode/mips.h: Complement change made to opcodes and remove
	references to the `g' regular MIPS ISA operand code.