MFC r268633, r271046:

  Fixes and enhancements for the if_cgem driver...

  - miibus fixes as suggested by Yonghyeon Pyun.
  - enable VLAN MTU support.
  - fix a few WITNESS complaints in cgem_attach().
  - have cgem_attach() properly init the ifnet struct before calling
    mii_attach() to fix panic when using e1000phy.
  - fix ethernet address changing.
  - fix transmit queue overflow handling.
  - tweak receive queue handling to reduce receive overflows.
  - bring out MAC statistic counters to sysctls.
  - add e1000phy to config file.
  - implement receive hang work-around described in reference guide.
  - change device name from if_cgem to cgem to be consistent with other
    interfaces.

  Fix the Zedboard/Zynq ethernet driver to handle media speed changes so
  that it can connect to switches at speeds other than 1gb.