default or selected kernelversion does not have config value CONFIG_BFIN_MAC_RMII.
Result is shown for kernelversion 5.4.229

RMII PHY Interface

configname: CONFIG_BFIN_MAC_RMII

Linux Kernel Configuration
└─>Device Drivers
└─>Network device support
└─>Ethernet (10 or 100Mbit)
└─>RMII PHY Interface
In linux kernel since version 2.6.24 (release Date: 2008-01-24)  
Use Reduced PHY MII Interface