ACPI / dock: avoid initializing acpi_dock_notifier_list multiple times

Function dock_add() will be called multiple times if there are
multiple dock stations, which causes acpi_dock_notifier_list to be
initialized multiple times.

To avoid that, move the initialization of acpi_dock_notifier_list
from dock_add() to acpi_dock_init().

[rjw: Changelog]
Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>