The i.MXRT1060 Reference Manual refers to RGMII Mode for Ethernet. The chapter is also titled "10/100 /1000 -Mbps Ethernet MAC (ENET)". Is that a typo? Gigabit Ethernet would be nice, but I don't think it has that. But if it had an RGMII interface, I would use it, even if gigabit speeds weren't supported (some 10/100 PHY's have RGMII interfaces).
I have since changed my mind about what I had said. Some 10/100 PHYs have an RGMII interface for processors that only have an RGMII interface. But I don't think there is any advantage to RGMII over RMII if all you want is 10/100. It is too bad they didn't include GigE in the i.MX RT.