Bluetooth: Add Device Unpaired mgmt event

This patch add a new Device Unpaired mgmt event. This will be sent to
all mgmt sockets except the one that requested unpairing (that socket
will get a command complete instead). The event is also reserved for
future SMP updates where a remote device will be able to request pairing
revocation from us.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>