macb: Add multicast capability

Add multicast capability to Atmel ethernet macb driver.

Signed-off-by: Patrice Vilchez <patrice.vilchez@rfo.atmel.com>
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>