ip17x.4: Initial manual page

Add a stub manual only saying what it is, how to include it, what is
supported, and since when. This plugs the ip17x driver into the manual
subsystem and the release hardware notes.

MFC to: 14.3
MFC after: 3 days
Reviewed by: adrian, carlavilla
Approved by: carlavilla (mentor)
Approved by: re (cperciva)
Differential Revision: https://reviews.freebsd.org/D50411

(cherry picked from commit f750a114d2c50c4d25d304dbf1dabfd3ae3aa013)
(cherry picked from commit ded154a1df97e3340f129394cbcaa1a50439d1bf)