net: dsa: mv88e6xxx: add G1 helper for ageing time

All Marvell switch chips from (88E6060 to 88E6390) have a ATU Control
register containing bits 11:4 to configure an ATU Age Time quotient.

However the coefficient used to calculate the ATU Age Time vary with the
models. E.g. 88E6060, 88E6352 and 88E6390 use respectively 16, 15 and
3.75 seconds.

Add a age_time_coeff to the info structure to handle this and a Global 1
helper to set the default age time of 5 minutes in the setup code.

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>