ACPI / dock: Associate dock platform devices with ACPI device objects

To allow user space to check which ACPI device object the dock
station is represented by, make acpi_dock_add() indicate to
platform_device_register_full() which ACPI device object should
be the companion of the new platform device.

This also ensures that the ACPI device object in question will
not go away while the dock platform device is present (which is
always).

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