greybus: hd/es2: add cport_clear callback and ARPC

Add a host-device cport_clear callback, which will be called as part
of the new connection tear-down sequence to reset the CPort state.

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>