Bluetooth: Add mgmt support for LE Secure Connections LTK types

We need a dedicated LTK type for LTK resulting from a Secure Connections
based SMP pairing. This patch adds a new define for it and ensures that
both the New LTK event as well as the Load LTKs command supports it.

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