discover/yaboot: Remove known_names list

The known_names list only duplicates the arg matching we do in the body
of the parser, and so introduces a problem when the array becomes out of
sync.

We drop the priority of the "unknown name" messages to pb_debug, as this
isn't really imporant unless we're debugging.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>