RGMII and RMII backwards compatibility
From HP's specifications on RGMII and TI's description of RMII mode, RMII is a subset of the RGMII pins, dropping 2 each of Tx and Rx. Thus, RMII operates at 50MHz for 100Mb operation while RGMII operates at 25MHz for 100Mb operation.

Looking a Broadcom switch product, it has a port that supports both RGMII and RMII. I suggest consulting your iMX6 SoM datasheet and processor datasheet to see if that RGMII port can be configured in RMII mode.