greybus: es2: Implement APBridgeA RPC (ARPC)

Implement ARPC. In first time, we are going to use it to
implement new request but the goal is to update all existing vendor
request to use ARPC.

In addition, Convert the current USB Vendor request for CPort Reset
to ARPC so that we can be sure that the port has been fully reset
by the time the request completes.

Testing Done:
  AP can reset APBA Cports by using the ARPC command.
  In addition, tested with a hacked firmware that cause error during
  the Cport reset, and Greybus printed the error "failed to reset cport".

Signed-off-by: Alexandre Bailon <abailon@baylibre.com>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Alex Elder <elder@linaro.org>