This is really a message for the SiFive documentation team.
After much searching, I found a set of IOF mappings for the FE310-G002 (in the -G002 manual). I’m going to assume that things are similar for the G000, namely:
UART and SPI pins are all IOF0 (IOF_SEL=0)
PWM pins are all IOF1 (IOF_SEL=1)
Maybe update the -G000 manual to include these? I was really going crazy trying to find this info! I even tracked down a copy of the Freedom E300 Platform manual hoping to find my answers there, but alas, no!