net: dsa: mv88e6xxx: add ATU setup helper

Move the configuration of the default ageing time in a new
mv88e6xxx_atu_setup function.

That function will be extended later to contain all ATU related
configuration bits.

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>