ACPI / hotplug: Add .fixup() callback to struct acpi_hotplug_context

In order for the ACPI dock station code to be able to use the
callbacks pointed to by the ACPI device objects' hotplug contexts
add a .fixup() callback pointer to struct acpi_hotplug_context.
That callback will be useful to handle PCI devices located in
dock stations.

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