Support for Firebox M270 and the Intel X553 MDIO

I’m looking at running FreeBSD on an old Firebox M270. 
It has a C3558 CPU with integrated X553 LAN controller which connects over MDIO to a Marvell 88E6190 switch.
Out of the box the X553 backplane is detected but there doesn’t seem to be any attempt at probing the MDIO for connected devices.

I’ve played around with the Intel ixgbe drivers, compiling the kernel etc to see if I can figure it out but I’m really struggling to understand what needs to be done.

At this stage I’m reaching out to the community to see if anyone can clarify something for me….are there some fundamental changes or additions required to the drivers to make this work, or is it something that should work already but needs some options enabling or configurations tweaking. I’m reluctant to put more time into trawling through the code if it’s a ‘simple’ configuration issue.

Any pointers would be very welcome.
Thank you.