[POWERPC] Document local bus nodes in the device tree, and update cuboot-pq2.

The localbus node is used to describe devices that are connected via a chip
select or similar mechanism.  The advantages over placing the devices under
the root node are that it can be probed without probing other random things
under the root, and that the description of which chip select a given device
uses can be used to set up mappings if the firmware failed to do so in a
useful manner.

cuboot-pq2 is updated to match the binding; previously, it called itself
chipselect rather than localbus, and used phandle linkage between the
actual bus node and the control node (the current agreement is to simply use
the fully-qualified address of the control registers, and ignore the overlap
with the IMMR node).

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>