[etherswitch] add ukswitch hint that is phy offset at mdio register

This patch allows to specify PHY register offset for ukswitch. For instance,
switch MAICREL KS8995XA connected via MDIO to SoC, but PHY register starts
at 1. So hint for this case is: hint.ukswitch.0.phyoffset=1

No change/effect if hint is not set.

Submitted by: Hiroki Mori <yamori813@yahoo.co.jp>
Reviewed by: adrian, mizhka
Approved by: adrian(mentor)
Differential Revision: https://reviews.freebsd.org/D8584