[POWERPC] cell: add support for proper device-tree

This patch adds support for a proper device-tree.
A porper device-tree on cell contains be nodes
for each CBE containg nodes for SPEs and all the
other special devices on it.
Ofcourse oldschool devicetree is still supported.

Signed-off-by: Christian Krafft <krafft@de.ibm.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>