[POWERPC] Xilinx: Update compatible to use values generated by BSP generator.

Mainly, this involves two changes:
1) xilinx->xlnx (recognized standard is to use the stock ticker)
2) In order to have the device tree focus on describing what the
hardware is as exactly as possible, the compatible strings contain the
full IP name and IP version.

Signed-off-by: Stephen Neuendorffer <stephen.neuendorffer@xilinx.com>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>