Add the ability to group sysfs nodes behind a firmware feature
check.  This way non-present sysfs attributes are omitted on
older firmware, which does not have newer features.

This will be used in the upcoming patches which adds more
features to the timecard.
Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>