greybus: es2: implement the fct flow control requests

Implement the control requests enabling/disabling the flow of FCT on APBA.

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