[POWERPC] Xilinx: Update booting-without-of.

This now better describes what the UBoot device tree generator
actually does.  In particular:

1) Nodes have a label derived from the device name, and a node name
derived from a generic version of the device type, e.g. 'ethernet',
'serial', etc.

2) Usage of compound nodes (representing more than one device in the
same IP) which actually works.  This requires having a valid
compatible node, and all the other things that a bus normally has.
I've chosen 'xlnx,compound' as the bus name to describe these compound
nodes.

In addition, I've clarified some of the language relating to how mhs
nodes should be represent in the device tree.

Signed-off-by: Stephen Neuendorffer <stephen.neuendorffer@xilinx.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>