[POWERPC] Add MPC8641 HPCN Device Tree Source file.

As per list discussion, let's add device tree source files
under powerpc/boot/dts.  If nothing else, it is a starting point.

Signed-off-by: Jon Loeliger <jdl@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>