I am investigating what it would take to add ethernet support to the KSDK for a custom board. I see that fsl_enet_rtcs_adapter.c requires fsl_phy_driver.h/.c to provide the low level PHY interface functions. For the demo projects, this lives in boards/common/phyksz8041. Looking at the code, none of this seems terribly hardware specific. It seems to boil down to a few SMI and ENET HAL calls, so I'm not too worried about porting to my application.
That said, is there any documentation that defines exactly what functions a phy driver needs to implement and what exactly is required of it?