boards/hifive1b: change clock Kconfig parameter to always have a value
boards/hifive1b: change clock Kconfig parameter to always have a value

As clock.c uses C conditionals for its code, the configuration macros
should always be defined. This is done by moving the dependencies to a
condition in the prompt, making them configurable only when applicable.

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