ACPI / dock: Use ACPI device object pointers instead of ACPI handles

Rework the ACPI dock station driver to store ACPI device object
pointers instead of ACPI handles in its internal data structures.

The purpose is moslty to make subsequent simplifications possible,
but also this allows the overall code size to be reduced slightly.

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