net: dsa: mv88e6xxx: add port base address to info

The switch ID is located at address 0x3 of every Port Registers bank.

But not all Marvell switches have their Port Registers SMI Addresses
starting at 0x10. 88E6060 starts at 0x8 and 88E6390 starts at 0x0.

Add this data in the info structure and use it in the detection 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>