Bluetooth: Add support for reading security information

To allow userspace to make correcty security policy decision, the kernel
needs to export a few details of the supported security features and
encryption key size information. This command exports this information
and also allows future extensions if needed.

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