powerpc/pci: Hide pci_device_from_OF_node() for non-powermac code

Function pci_device_from_OF_node() is used only in powermac code. So
hide it from all other platforms.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220706104308.5390-2-pali@kernel.org