net: systemport: Add netconsole support

Implement a poll controller for netconsole which invokes the RX
interrupt handler to poll for incoming packets, and cleans up all TX
queues by invoking the TX interrupt handler.

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