net: dsa: mv88e6xxx: add a global2_addr info flag

Similarly to global1_addr, add a global2_addr member in the info
structure to describe the presence of the Global 2 Registers.

This allows us to get rid of the MV88E6XXX_FLAG_GLOBAL2 flag.

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>