greybus: connection: add api to {en,dis}able unipro fct flow

In order to support mailbox-free control cport init on the bridges the AP must
be able to enable/disable the flow of unipro fct tokens. Add a new API that
will enable or disable on APBA the flow of fct tokens.

Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Fabien Parent <fparent@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>