cpu/samd5x: use ONDEMAND bit to run clocks on demand
cpu/samd5x: use ONDEMAND bit to run clocks on demand

Set the ONDEMAND bit so clocks are only run if they have a user configured.

This saves 390 µA on same54-xpro.

 examples/default:

    before: 3.88 mA
    after : 3.49 mA

 examples/gnrc_networking: (with REB215-XPRO EXT3)

    before: 13.29 mA
    after : 12.9  mA

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