greybus: bundle.h: add a private field to struct gb_bundle

The gb_bundle structure needs a private field if we are going to be able
to replace the gb_connection device with this one for all use cases.

Ideally we could use the private pointer within the struct device, but
for now let's just try to mirror how people were using the gb_connection
structure to make the patches simpler, and the logic the same.

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