[POWERPC] Move PCI nodes to be sibilings with SOC nodes

Updated the device trees to have the PCI nodes be at the same level as
the SOC node.  This is to make it so that the SOC nodes children address
space is just on chip registers and not other bus memory as well.

Also, for PCIe nodes added a P2P bridge to handle the virtual P2P bridge
that exists in the PHB.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>