ACPI: export acpi_pci_root and friends

We can simplify ACPI drivers if we can tell whether a handle is an
ACPI PCI root or not.

Reviewed-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>