Bluetooth: Add support for LE SC key generation

As the last step of the LE SC pairing process it's time to generate and
distribute keys. The generation part is unique to LE SC and so this
patch adds a dedicated function for it. We also clear the distribution
bits for keys which are not distributed with LE SC, so that the code
shared with legacy SMP will not go ahead and try to distribute them.

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