net: lan966x: Add support to offload the forwarding.

This patch adds basic support to offload in the HW the forwarding of the
frames. The driver registers to the switchdev callbacks and implements
the callbacks for attributes SWITCHDEV_ATTR_ID_PORT_STP_STATE and
SWITCHDEV_ATTR_ID_BRIDGE_AGEING_TIME.
It is not allowed to add a lan966x port to a bridge that contains a
different interface than lan966x.

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