Also add the code for:
1. Enabling the clock gate for the Ethernet module of the MCU,
2. Pin configuration for RMII.
3. Disabling the MPU (the Ethernet module will be unable to work with
     the SRAM otherwise.)

The pull-down resistor for the RXER pin is enabled, because this input
pin is not connected to the PHY on the TWR-K60N512 board by default.