I know the procedure to enable an external RTC, but I just wonder which are the available supported chip (in config.txt and to use with modprobe).
Where can I find such a list?
Read /boot/overlays/README
particularly the i2c-rtc
section.
DO NOT use modprobe
use Device Tree!
Pick any of the listed modules.
I recommend ds3231
which is a good performer and works from 3.3V, although many of the others work well.