Rework BGX detection to support both new and old firmware

Improve existing BGX detection and adjust it to support both
new and older ThunderX firmwares. Match BGX FDT nodes by name
and reg. Match PHY instances by qlm-mode and name.
Tested on Firmware Version: 2016-09-30 09:12:11

Obtained from: Semihalf
Differential Revision: https://reviews.freebsd.org/D9863