net: dsa: support switchdev ageing time attr

Add a new function for DSA drivers to handle the switchdev
SWITCHDEV_ATTR_ID_BRIDGE_AGEING_TIME attribute.

The ageing time is passed as milliseconds.

Also because we can have multiple logical bridges on top of a physical
switch and ageing time are switch-wide, call the driver function with
the fastest ageing time in use on the chip instead of the requested one.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>