I followed this post, but the RTC still did not appear in /dev
viewtopic.php?f=31&t=8243
The only step that was missing was adding the device to the Device tree:
Edit /boot/config.txt and add the following 3 lines:
Raspbery Pi b
device_tree=bcm2708-rpi-b.dtb
device_tree_param=i2c1=on
device_tree_param=spi=on
or Raspberry Pi B plus
device_tree=bcm2708-rpi-b-plus.dtb
device_tree_param=i2c1=on
device_tree_param=spi=on
After that the Piface clock worked fine. It tends to gain 10-20 seconds per day, so I update the clock every 24 hours by executing "hwclock -w". Hope this helps somebody.