greybus: es2: Add a new bulk in endpoint for APBridgeA RPC

Add a new bulk in endpoint in order to get RPC status
from APbridgeA without to have to poll on control endpoint.

So the new endpoint layout is:
EP0: control EP
EP1 and EP2: muxed endpoints (bulk in and out)
EP3 to EP14: direct muxed endpoints (bulk in and out)
EP15: ARPC bulk in endpoint

Note:
This patch is allocating ARPC URBs but does nothing with them.
The following patch will use them.

Testing Done:
  Tested with an APBridgeA enumerating 16 endpoints.
  Kernel doesn't print Not enough endpoints found in device, aborting!

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