In the i.MX 6ULL Applications Processor Reference Manual, Rev. 1 throughout table 22-1 and 22-2 there are many mentions of RGMII which I don't believe is supported in the chip. The word RGMII only appears in these tables.
In the i.MX 6ULL Applications Processors for Industrial Products, Rev. 1.2, 11/2017 in figure 67, 68 RGMII_TXD, RGMII_RXD are shown in the diagrams as pin names.