[POWERPC] Xilinx: Add correct compatible list for device tree bus bindings.

Includes both flavors of plb, opb, dcr, and a pseudo 'compound' bus
for representing compound peripherals containing more than one logical
device.

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