Add deprecation notices to ctau and cx drivers
These support outdated or obsolete ISA WAN (T1/E1) sync serial cards,
and these drivers haven't really been touched (other than in tree-wide
sweeps to keep them building) for 15+ years.
Related PCI devices ce and cp are still in the tree, with deprecation
proposed in D23928.
MFC after: 1 week
Sponsored by: The FreeBSD Foundation