greybus: connection: add helper to create control connections

Add dedicated helper to create control connections.

This will allow us to simplify the generic (dynamic) interface.

Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>