$DF13/57107/MMC+3:      Identification Register

  On Read:   #$64 when Bit 1 of $DF11 is 0
             #$01 when Bit 1 of $DF11 is 1 and REV A hardware is used
             #$02 when Bit 1 of $DF11 is 1 and REV B hardware is used

  On Write:  Write #$55 & #$AA into this register to unlock Bit 3 of $DF11
             Write #$0A & #$1C into this register to re-enable MMC64 hardware