powerpc/virtex: add dts file for ML507 reference design

This new file adds support for the ML507 reference design.  The ML507
uses the Virtex 5 FXT FPGA which embeds a ppc440 core.

Signed-off-by: John Linn <john.linn@xilinx.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>