cpu/samd5x: enable buck voltage regulator when possible
cpu/samd5x: enable buck voltage regulator when possible

When an external oscillator is used and the internal fast oscillators
are off, we can enable the buck converter if the board supports it.

authmillenon/RIOT,basilfx/RIOT,kaspar030/RIOT,OTAkeys/RIOT,miri64/RIOT,basilfx/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,kYc0o/RIOT,basilfx/RIOT,miri64/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,ant9000/RIOT,jasonatran/RIOT,jasonatran/RIOT,kYc0o/RIOT,kYc0o/RIOT,miri64/RIOT,kYc0o/RIOT,miri64/RIOT,kaspar030/RIOT,OTAkeys/RIOT,kaspar030/RIOT,kaspar030/RIOT,ant9000/RIOT,kaspar030/RIOT,ant9000/RIOT,OlegHahm/RIOT,basilfx/RIOT,authmillenon/RIOT,miri64/RIOT,jasonatran/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,ant9000/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,authmillenon/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,basilfx/RIOT,ant9000/RIOT,OTAkeys/RIOT,jasonatran/RIOT