ACPI: convert acpi_bus_scan() to operate on an acpi_handle

This patch changes acpi_bus_scan() to take an acpi_handle rather than an
acpi_device pointer.  I plan to use acpi_bus_scan() in the hotplug path,
and I'd rather not assume that notifications only go to nodes that already
have acpi_devices.

This will also help remove the special case for adding the root node.  We
currently add the root by hand before acpi_bus_scan(), but using a handle
here means we can start the acpi_bus_scan() directly with the root even
though it doesn't have an acpi_device yet.

Note that acpi_bus_scan() currently adds and/or starts the *children* of
its device argument.  It doesn't do anything with the device itself.

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