ACPI / hotplug / PCI: Hotplug notifications from acpi_bus_notify()

Since acpi_bus_notify() is executed on all notifications for all
devices anyway, rename acpi_hotplug_notify_cb() to acpi_system_notify()
and call it directly from acpi_bus_notify() instead of installing
notify handlers pointing to it for all hotplug devices.

This change reduces both the size and complexity of ACPI-base device
hotplug code.  Moreover, since acpi_system_notify() only does
significant things for devices that either have an ACPI scan handler,
or have a hotplug context with .eject() defined, and those devices
had notify handlers pointing to acpi_hotplug_notify_cb() installed
before anyway, this change shouldn't modify functionality.

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