Core/system clock rate: 100 MHz
Peripheral clock rate:   50 MHz
FlexBus clock rate:      50 MHz
Flash clock rate:        25 MHz

The PLL takes the 50 MHz on-board external clock from the EXTAL pin and
feeds the MCGOUTCLK with a 100 MHz clock.

On the Kinetis MCU, we have to sequentally switch between 3 clocking
modes until we can enter the desired PEE mode (PLL Engaged External
mode).

The `KINETIS_MCG_C2_EREFS_MSK` and `KINETIS_MCG_C2_HGO_MSK` bits should
be cleared, otherwise the Ethernet module of the MCU cannot send nor
receive packets.

Testing:
    `udelay(1000)` works correctly (tested by blinking the green LED.)