PPC: remove dead boards (AMX860, c2mon, ETX094, IAD210, LANTEC, SCM)

These boards have long reached EOL, and there has been no indication
of any active users of such hardware for years.  Get rid of the dead
weight.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Wolfgang Grandegger <wg@denx.de>