Bluetooth: Add definitions for new link key types

With the introduction of Secure Connections, the list of link key types
got extended by P-256 versions of authenticated and unauthenticated
link keys.

To avoid any confusion the previous authenticated and unauthenticated
link key types got ammended with a P912 postfix. And the two new keys
have a P256 postfix now. Existing code using the previous definitions
has been adjusted.

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