docs: ABI: Document new timecard sysfs nodes.

Add sysfs nodes for the frequency generator and signal counters.

Update SMA selector lists for these, and also add the new
'None', 'VCC' 'GND' selectors.

Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>