[POWERPC] Add device tree for Ebony

Add a device tree for the Ebony evaluation board (440GP based).  This
tree is not complete or finalized.  This tree needs a version of dtc
recent enough to include reference-to-labels to process.

Signed-off-by: David Gibson <dwg@au1.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>