Bluetooth: Add new structures for handling SMP Long Term Keys

This includes a new list for storing the keys and a new structure used
to represent each key.

Some notes: authenticated is used to identify that the key may be used
to setup a HIGH security link. As the same list is used to store both
the STK's and the LTK's the type field is used so we can separate
between those two types of keys and if the key should be used when
in the master or slave role.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@openbossa.org>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>