greybus: hd/es2: add cport_connected callback and ARPC

Add a host-device cport_connected callback, which will be called after a
connection has been created and that can be used by the host-device
driver to make sure its internal state is updated to match the CPort
attributes set by the SVC.

This callback will eventually replace the cport_features_enable
callback.

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