Bluetooth: Add mgmt HCI channel registration API

This patch adds an API for registering HCI channels with mgmt-like
semantics. For now the only user will be HCI_CHANNEL_CONTROL, but e.g.
6lowpan is intended to use this as well in the future.

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