net: dsa: mv88e6xxx: add flags for FID registers

Add flags to describe the presence of Global 1 ATU FID register (0x01)
and VTU FID register (0x02), instead of checking families.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>