net: dsa: mv88e6xxx: add cap for Priority Override

Add flags and helpers to describe the presence of Priority Override
Table (POT) related registers and simplify the setup of Global 2.

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>