Enable in-band link management on A388-Clearfog board

This patch adds in-band link management over SGMII of the
SFP transceiver on Armada-388-Clearfog board.

Submitted by: Marcin Wojtas <mw@semihalf.com>
Obtained from: Semihalf
Sponsored by: Netgate
Reviewed by: loos
Differential revision: https://reviews.freebsd.org/D10708