greybus: hd/es2: add cport_quiesce callback and ARPC

Add a host-device cport_quiesce callback, which will be called as part
of the new connection tear-down sequence to disable flow control after
first making sure that enough peer buffer space is available for the
next messages about to be sent.

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>