Its a different MCU altogther (STM32F207), where as photon’s is STM32F205 without ethernet MAC. Yes it is RMII compatible. What you find in the hal directory are the wiced libraries. You ought to download the wiced stack and make some modifications as to hook the ethernet driver of STM32, compile/link and include these modifed library in the hal.
If you plan on using W5100, follow the steps and procedure what Travis has suggested.
In your case, initialise the W5100 and get the ip via dhcp or use static ip for now and make use of socket hal files to setup the tcp and send/receive data and not the one provided by w5100, instead make use of particle socket drivers.