net: dsa: mv88e6xxx: lookup switch name

All the mv88e6xxx drivers use the exact same code in their probe
function to lookup the switch name given its ID. Thus introduce a
mv88e6xxx_switch_id structure and a mv88e6xxx_lookup_name function in
the common mv88e6xxx code.

In the meantime make __mv88e6xxx_reg_{read,write} static since we do not
need to expose these low-level r/w routines anymore.

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