Attempt to look up the MAC address in Open Firmware on systems that
support it. On SPARC resort to using the IDPROM if no OF address is
found.

Signed-off-by: Martin K Petersen <martin.petersen@oracle.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>