* In order to switch the MCG to the PLL Engaged External mode (PEE
mode), it must pass the FLL Bypassed External mode (FBE mode) as an
intermediate step. See section `25.4.1 MCG Mode State Diagram` on page
656 of the K70 Reference Manual.
 * Enable and use the RTC clock for FLL when switching to the
FLL Bypassed External mode (FBE mode). We do this in `clock_fei_to_fbe()`
in `u-boot/cpu/arm_cortexm3/kinetis/clock.c` before switching to
the FBE mode. This is required, because:
   * In order to switch to the PLL Engaged External mode (PEE mode), we
must pass the FLL Bypassed External mode (FBE mode).
   * This FBE mode requires that there is a working FLL reference clock.
   * Only OSC0 clock (clock or oscillator at EXTAL0/XTAL0) or RTC clock
(oscillator at EXTAL32/XTAL32) can be used as reference clock for FLL.
   * In the K**-SOM/DNI-ETH configuration, nothing is connected to
EXTAL0, therefore we have to use the RTC clock as reference clock
for FLL in the FBE mode.
 * Use RTC for FLL reference clock only on K**-SOMs, but not for
TWR-K70F120M. To do that, we add a new U-Boot configuration option
`KINETIS_FLLREF_RTC` that will control usage of RTC for FLL reference
clock and define this configuration option only in
`u-boot/include/configs/k70-som.h`.
 * Use in-MCU 20pF oscillator load.