target-mips: add microMIPS32 R6 opcode enum

Add microMIPS32 Release 6 opcode enum.
Remove RI checking for pre-R6 reserved opcode.

Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
