arc_emac: implement promiscuous mode and multicast filtering

This patch implements the set_rx_mode function to enable/disable
promiscuous or all-multicast modes and to update the multicast
filtering list of the device.

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>