net: stmmac: enable multiple buffers

This patch creates 2 new structures (stmmac_tx_queue and stmmac_rx_queue)
in include/linux/stmmac.h, enabling that each RX and TX queue has its
own buffers and data.

Signed-off-by: Joao Pinto <jpinto@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>