mtkswitch.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 mtkswitch 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/D50412

(cherry picked from commit 37f00bc257d0e785fad1d0ffbd7dcbf09e863417)
(cherry picked from commit f31ac06711e69aed7cb4b38d8eb2aba2d7149ed8)