pci: remove ob_pci_initialize() and ob_pci_empty_node

The ob_pci_initialize() function can be removed as it is empty, whilst
ob_pci_empty_node is no longer required since the existing fallback code
will always apply the ob_pci_simple_node bindings if no callback is
configured.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>