MIPS16/opcodes: Free up `M' operand code

MIPS16/opcodes: Free up `M' operand code

The `M' and `m' MIPS16 operand codes are functionally the same, denoting
a 7-bit register list that is encoded the same way for both SAVE and
RESTORE.  Use `m' for both instructions then, making `M' available for a
different use.

	opcodes/
	* mips16-opc.c (decode_mips16_operand) <'M'>: Remove case.
	(mips16_opcodes): Replace `M' with `m' for "restore".

	include/
	* opcode/mips.h: Remove `M' operand code.