PCI/portdrv: Rename pm_iter() to pcie_port_device_iter()

PCI/portdrv: Rename pm_iter() to pcie_port_device_iter()

Rename pm_iter() to pcie_port_device_iter() and make it visible outside
CONFIG_PM and portdrv_core.c so it can be used for pciehp slot reset
recovery.

[bhelgaas: split into its own patch]
Link: https://lore.kernel.org/linux-pci/08c046b0-c9f2-3489-eeef-7e7aca435bb9@gmail.com/
Link: https://lore.kernel.org/r/251f4edcc04c14f873ff1c967bc686169cd07d2d.1627638184.git.lukas@wunner.de
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>