switchdev: add bridge ageing_time attribute

Setting the stage to push bridge-level attributes down to port driver so
hardware can be programmed accordingly.  Bridge-level attribute example is
ageing_time.  This is a per-bridge attribute, not a per-bridge-port attr.

Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>