ACPI: use _HID when supplied by root-level devices

Previously, we assumed the only Device object immediately below the root
was the \_SB Scope (which the ACPI CA treats as a Device), so we forced
the HID of all such objects to ACPI_BUS_HID ("LNXSYBUS").

However, there are DSDTs that supply root-level Device objects with _HIDs.
This patch makes us pay attention to those _HIDs and only add the synthetic
ACPI_BUS_HID for root-level objects that do not supply their own _HID.

For example, this DSDT: https://bugzilla.kernel.org/show_bug.cgi?id=15605


contains:

    Scope (_SB) {
	...
    }
    Device (AMW0) {
	Name (_HID, EisaId ("PNP0C14"))
	...
    }

and we should use "PNP0C14" for the AMW0 device, not "LNXSYBUS".

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Acked-by: Zhang Rui <rui.zhang@intel.com>
Tested-by: Yong Wang <yong.y.wang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>