Bluetooth: Add support for handling LE SC user response

With LE SC, once the user has responded to the numeric comparison it's
time to send DHKey check values in both directions. The DHKey check
value is generated using new smp_f5 and smp_f6 cryptographic functions.
The smp_f5 function is responsible for generating the LTK and the MacKey
values whereas the smp_f6 function takes the MacKey as input and
generates the DHKey Check value.

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