net: dsa: mv88e6xxx: describe Multi-chip registers

Add flags to describe the presence of SMI Command and Data registers
used to indirectly access internal SMI devices registers when the switch
SMI address is not zero.

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