IBM zSystems: mgrk, mg first operand requires register pair

opcodes/

	* s390-opc.c (INSTR_RRF_R0RER): New instruction type.
	(MASK_RRF_R0RER): Define mask for new instruction type.
	* s390-opc.txt: Use RRF_R0RER for mgrk and RXY_RERRD for mg.