cpu/fe310: run RTT at 1 Hz if RTC is selected
cpu/fe310: run RTT at 1 Hz if RTC is selected

The RTC on the fe310 is emulated using the RTT.
This only works if the RTT frequency is 1 Hz, so default to that
value in case `periph_rtc` is selected.

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