PNP: add interface to retrieve ACPI device from a PNPACPI device

Add pnp_acpi_device(pnp_dev), which takes a PNP device and returns the
associated ACPI device (or NULL, if the device is not a PNPACPI device).

This allows us to write a PNP driver that can manage both traditional
PNPBIOS and ACPI devices, treating ACPI-only functionality as an optional
extension.

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