ACPI / scan: Use direct recurrence for device hierarchy walks

Rework acpi_bus_trim() and acpi_bus_device_attach(), which is
renamed as acpi_bus_attach(), to walk the list of each device
object's children directly and call themselves recursively for
each child instead of using acpi_walk_namespace().  This
simplifies the code quite a bit and avoids the overhead of
callbacks and the ACPICA's internal processing which are not
really necessary for these two routines.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>