powerpc/pci: Make both ppc32 and ppc64 use sysdata for pci_controller

Currently, ppc32 uses sysdata for the pci_controller pointer, and
ppc64 uses it to hold the device_node pointer.  This patch moves the
of_node pointer into (struct pci_bus*)->dev.of_node and
(struct pci_dev*)->dev.of_node so that sysdata can be converted to always
use the pci_controller pointer instead.  It also fixes up the
allocating of pci devices so that the of_node pointer gets assigned
consistently and increments the ref count.

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