keystone2: net: add mcast function to keyston2 Ethernet driver

keystone2: net: add mcast function to keyston2 Ethernet driver

The MCAST_TFTP support requires that network drivers has mcast functon
implemented. This commit adds dummy keystone2_eth_bcast_addr() to meet
the requirement. As far as the driver doesn't use ALE and doesn't filter
any incoming packets, the function is empty.

Signed-off-by: Vitaly Andrianov <vitalya@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>