greybus: connection: add offloaded connection flag

Add an offloaded connection flag, which is used to mark a connection as
offloaded and prevent drivers from initiating operation over it.

This will be used for the audio and camera data connections.

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