Using bisection, I've determined that this was introduced in the following xenial commit:
commit 69747b70c731cc7081e7d13a0f536bb451f5a385 Author: Iyappan Subramanian <email address hidden> Date: Mon Jul 25 17:12:42 2016 -0700
drivers: net: xgene: Enable MDIO driver
Using bisection, I've determined that this was introduced in the following xenial commit:
commit 69747b70c731cc7 081e7d13a0f536b b451f5a385
Author: Iyappan Subramanian <email address hidden>
Date: Mon Jul 25 17:12:42 2016 -0700
drivers: net: xgene: Enable MDIO driver