cpu/saml21: enable buck voltage regulator when possible
cpu/saml21: enable buck voltage regulator when possible

Switch from the on-chip LDO to the on-chip buck voltage regulator
when not fast internal oscillators are used.

On `saml21-xpro` with `examples/default` this gives

**before:** 750 µA
** after:** 385 µA

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