greybus: legacy: refactor legacy-connection handling

Add wrapper structure for legacy connections and add helpers to create
and destroy legacy connections.

This is a step in removing legacy connection fields from the core
structures.

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>