Documentation: PM: Update platform_pci_wakeup_init() reference

platform_pci_wakeup_init() was removed by d2e5f0c1 ("ACPI / PCI: Rework
the setup and cleanup of device wakeup") but was still mentioned in the
documentation.

Update the doc to refer to pci_acpi_setup(), which does the equivalent
work.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Dhruva Gole <d-gole@ti.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>