powerpc/pci: Use generic pci_mmap_resource_range()

Commit f7195824 ("PCI: Add pci_mmap_resource_range() and use it for
ARM64") added this generic function with the intent of using it everywhere
and ultimately killing the old arch-specific implementations.

Remove the powerpc-specific pci_mmap_page_range() and use the generic
pci_mmap_resource_range() instead.

Powerpc can mmap I/O port space, so supply the powerpc-specific
pci_iobar_pfn() required to make that work.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
[bhelgaas: changelog]
Signed-off-by: Bjorn Helgaas <helgaas@kernel.org>