powerpc/4xx: update ml507 .dts file to release reference design

This patch updates the Xilinx ML507 device tree to match the released
ML507 powerpc reference design (ml507_ppc440_emb_ref).  This patch is
needed to boot Linux on the ML507 powerpc reference design without
manually generating and tweaking a device tree from the project directory.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>