ACPI: Introduce acpi_is_root_bridge()

Returns whether an ACPI CA node is a PCI root bridge or not.

This API is generically useful, and shouldn't just be a hotplug function.

The implementation becomes much simpler as well.

Signed-off-by: Alex Chiang <achiang@hp.com>
Acked-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>