Changes to update the FreeBSD axgbe driver on-par to the Linux axgbe driver

[rajesh1.kumar_amd.com]

Summary

This patch has the preparatory changes to update the FreeBSD axgbe driver
on-par to the Linux axgbe driver.

This is the first patch of multiple changes that has been made. This patch has
the changes mainly in xgbe-mdio.c to move its functions to xgbe-phy-v1.c(so that
xgbe-mdio.c will be more like a wrapper code). xgbe-phy-v1 has the necessary
functions for the old version of hardware. If this changes looks good,
we can extend the code for the new versin of hardware (xgbe-phy-v2).

The current changes are done with the intent to not alter existing logics, but
just to split the code between xgbe-mdio.c and xgbe-phy-v1.c

Test Plan

I have the new hardware version, where the XGBE IP lies in PCI bus. We have the
pci driver written for it and verified with this changes also. But I don't have
the old version of hardware to verify this changes. So, need some help here to
verify and review the changes.

[rajesh1.kumar_amd.com]

Hi @andrew

This patch is just one part of the changes we made to the axgbe driver. If this looks good, we can update the other changes also one by one. Please let me know if I can split up this patch further for ease of review.