Circuit design for co-layout the XTAL and CPU clock output
I would recommend putting both circuits on the board, and where the clock input line to your PHY is, put a solder jumper. Then, you can essentially break the connection between the CPU and the PHY if you choose to use the crystal. You can also put solder jumpers between the crystal and the PHY, and if you choose to use the CPU clock output, then you can open the jumpers between the crystal and the PHY.