greybus: connection: remove 'struct device' from 'struct gb_connection'

We don't want this in the driver core, as nothing will be binding to it,
that's the job of a bundle.  So remove the struct device and use a kref
to handle reference counting instead.

Note, I don't think we really need a kref, but it keeps the lifetime the
same as before, and is the simplest change for now.  In the future it
might be easier to just attach all connections to the bundle and clean
them up when the bundle is removed.

Also remove the cport sysfs documentation as it's no longer relevant.

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>