cpu/samd5x: work around errata when (re-)initializing DFLL
cpu/samd5x: work around errata when (re-)initializing DFLL

When a previously disabled DFLL gets enabled again, the frequency will
be incorrect. Follow the procedure outlined in the errata sheet, section 2.8.3
to work around the issue.

This fixes wake from standby.

jasonatran/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,basilfx/RIOT,yogo1212/RIOT,yogo1212/RIOT,OlegHahm/RIOT,kYc0o/RIOT,jasonatran/RIOT,ant9000/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,yogo1212/RIOT,miri64/RIOT,basilfx/RIOT,ant9000/RIOT,yogo1212/RIOT,kaspar030/RIOT,authmillenon/RIOT,jasonatran/RIOT,ant9000/RIOT,OlegHahm/RIOT,kYc0o/RIOT,kaspar030/RIOT,kYc0o/RIOT,miri64/RIOT,ant9000/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,OlegHahm/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,basilfx/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,miri64/RIOT,ant9000/RIOT,miri64/RIOT,basilfx/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,jasonatran/RIOT,jasonatran/RIOT,authmillenon/RIOT,kYc0o/RIOT,authmillenon/RIOT,yogo1212/RIOT,authmillenon/RIOT,miri64/RIOT,kaspar030/RIOT,authmillenon/RIOT,authmillenon/RIOT,OTAkeys/RIOT,basilfx/RIOT