net: bcmgenet: core changes for supporting multiple Rx queues

1. Add struct bcmgenet_rx_ring to hold all necessary information
   for a single Rx queue.
2. Add bcmgenet_init_rx_queues() to initialize all Rx queues.
3. Modify bcmgenet_init_rx_ring() to initialize a single Rx queue.
4. Modify Rx interrupt path code to use per-queue data.
5. Modify bcmgenet_rx_refill() to use RxCB->bd_addr.

Signed-off-by: Petri Gynther <pgynther@google.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>