cpu/fe310: RTC: use rtc_mktime()
cpu/fe310: RTC: use rtc_mktime()

Use RTC helper functions instead of libc functions.
This gives us y2038 safety by the extended epoch and saves
a good chunk of memory:

picolibc mktime():

   text	   data	    bss	    dec	    hex	filename
  15048	    520	   2504	  18072	   4698	tests/periph_rtc/bin/hifive1/tests_periph_rtc.elf

rtc_mktime():

   text	   data	    bss	    dec	    hex	filename
   7632	     40	   2452	  10124	   278c	tests/periph_rtc/bin/hifive1/tests_periph_rtc.elf

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