powerpc/pci: Clean up direct access to sysdata by celleb platforms

We shouldn't directly access sysdata to get the device node to just
go get the pci_controller.  We can call pci_bus_to_host() for this
purpose.

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