pci: Add pci_get_devfn() to extract devfn from the fdt_pci_addr

This function will be used by the Marvell Armada XP/38x PCIe driver,
which is moved to DM right now. So let's extract the functionality
from pci_uclass_child_post_bind() to make it available.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>