Make tsec work with the device tree present on the RB800. The previous code
assumed that the MDIO bus was a direct child of the Ethernet interface. It
may not be and indeed on many device trees is not. While here, add proper
locking for MII transactions, which may be on a bus shared by several MACs.

Hardware donated by:	Benjamin Perrault
Be more flexible about which compatible strings to accept. This brings up
the PCI Express bus on the RB800 using the firmware device tree.
Rename the "bare" platform "mpc85xx", which is what it actually is, and
add actual platform probing based on PVR. Still needs a little more work:
in particular, the CCRS setup should move here.

Also turn "bare" into a truly bare platform that doesn't pretend to know how
to do anything except get the memory map. This should also be enhanced to
process the FDT reserved memory list, but that is for another day.
Set description string for VM_GUEST_HV (HyperV guest).
This fixes fallout from r256425.

Reported by:	Pavel Timofeev <timp87@gmail com>
Tested by:	Pavel Timofeev <timp87@gmail com>
Reviewed by:	Roger Pau Monnц╘
MFC after:	3 days