net: dsa: mv88e6xxx: add POT flag to 88E6390

The 88E6390 family clear the Priority Override Table the same way as
88E6352, thus add MV88E6XXX_FLAG_G2_POT to MV88E6XXX_FLAGS_FAMILY_6390.

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>