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).