ACPI: pci_root: pass acpi_pci_root to arch-specific scan

The acpi_pci_root structure contains all the individual items (acpi_device,
domain, bus number) we pass to pci_acpi_scan_root(), so just pass the
single acpi_pci_root pointer directly.

This will make it easier to add _CBA support later.  For _CBA, we need the
entire downstream bus range, not just the base bus number.  We have that in
the acpi_pci_root structure, so passing the pointer makes it available to
the arch-specific code.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Reviewed-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Len Brown <len.brown@intel.com>