Yes is possible to use LPC546x in RMII mode.
As you could check, in our schematic for the LPCxpresso546x,
the LAN8720A-CP PHY, which supports the RMII MAC interface, is implemented among the LPC54608 RMII interface.
Please note that in the schematic design the ENET_RX_DV pin of the LPC546x is connected to the CRS_DV pin of the PHY.
Best regards, Diego.