Bluetooth: Create SMP device structure for local crypto context

Every Bluetooth Low Energy controller requires a local crypto context
to handle the resolvable private addresses. At the moment this is just
a single crypto context, but for out-of-band data generation it will
require an additional. To facility this, create a struct smp_dev that
will hold all the extra information. This patch is just the refactoring
in preparation for future changes.

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