ACPI / ATA: Add hotplug contexts to ACPI companions of SATA devices

Modify the SATA subsystem to add hotplug contexts to ACPI companions
of SATA devices and ports instead of registering special ACPI dock
operations using register_hotplug_dock_device().

That change will allow the entire code handling those special ACPI
dock operations to be dropped in the next commit.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Aaron Lu <aaron.lu@intel.com>
Acked-by: Tejun Heo <tj@kernel.org>