Add DTS file for Solidrun ClearFog board

ClearFog is equipped with Marvell Armada 388 SoC, which is already
supported in FreeBSD.

Submitted by: Bartosz Szczepanek <bsz@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield
Differential revision: https://reviews.freebsd.org/D7326