Bluetooth: mgmt: Implement support for passkey notification

This patch adds support for Secure Simple Pairing with devices that have
KeyboardOnly as their IO capability. Such devices will cause a passkey
notification on our side and optionally also keypress notifications.
Without this patch some keyboards cannot be paired using the mgmt
interface.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Cc: stable@vger.kernel.org
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>