ACPI / PNP: skip ACPI device nodes associated with physical nodes already

Make pnpacpi_add_device() ignore ACPI device nodes already associated
with struct device objects representing physical devices.

In particular, this will prevent PNP device objects from being
created for ACPI device nodes already associated with platform
devices.

This change was originally proposed by Mika Westerberg.

[rjw: Modified the subject and changelog.]
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>