Bluetooth: Add handler function for receiving LE SC public key

This patch adds a handler function for the LE SC SMP Public Key PDU.
When we receive the key we proceed with generating the shared DHKey
value from the remote public key and local private key.

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