net: lan966x: Add function lan966x_mac_ip_learn()

Extend mac functionality with the function lan966x_mac_ip_learn. This
function adds an entry in the MAC table for IP multicast addresses.
These entries can copy a frame to the CPU but also can forward on the
front ports.
This functionality is needed for mdb support. In case the CPU and some
of the front ports subscribe to an IP multicast address.

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>