powerpc/pci: Implement pci_resource_to_user() with pcibios_resource_to_bus()
"User" addresses are shown in /sys/devices/pci.../.../resource and
/proc/bus/pci/devices and used as mmap offsets for /proc/bus/pci/BB/DD.F
files. For I/O port resources on powerpc, these are PCI bus addresses,
i.e., raw BAR values.
Previously pci_resource_to_user() computed the user address by subtracting
"hose->io_base_virt - _IO_BASE" from the resource start: