Fix MAC address configuration for VNIC

The FDT description is as follows:

• phy-handle, reg, qlm-mode, mac-address are under nodes in bgx0/1 node
• phy nodes (pointed by phy-handle) are under MDIO even though they may not be connected through to MDIO. In those nodes they do not contain MAC address or etc.

This commit changes parsing of the FDT nodes for BGX so that it can
obtain correct MAC address for a given PHY.

Reviewed by: wma
Obtained from: Semihalf
Sponsored by: Cavium
Differential Revision: https://reviews.freebsd.org/D5781