net: dsa: mv88e6xxx: Add helper for accessing port registers

There is a device coming soon which places its port registers
somewhere different to all other Marvell switches supported so far.
Add helper functions for reading/writing port registers, making it
easier to handle this new device.

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