Bluetooth: Add management command for Secure Connection Only Mode

With support for Secure Connections it is possible to switch the
controller into a mode that is called Secure Connections Only. In
this mode only security level 4 connections are allowed (with the
exception of security level 0 approved services).

This patch just introduces the management command and setting of the
right internal flags to enable this mode. It does not yet enforce it.

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