dma: dma.conf.5 as MLINK to dma.8

There's no separate man page for dma.conf, but the format is documented
in dma.8.

Reviewed by: bapt
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37944

(cherry picked from commit 45396fda8b73146d3980797d48704184a5aa99ff)