net: dsa: sja1105: Add support for configuring address ageing time

If STP is active, this setting is applied on bridged ports each time an
Ethernet link is established (topology changes).

Since the setting is global to the switch and a reset is required to
change it, resets are prevented if the new callback does not change the
value that the hardware already is programmed for.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>