net: dsa: mv88e6xxx: add a multi_chip info flag

Instead of relying on a bitmap flag, add a new multi_chip info flag to
describe the presence of the indirect SMI access though the two device
registers 0x0 and 0x1.

All remaining capabilities and flags are now unused. Remove the
mv88e6xxx_cap enum and the info flags bitmaps.

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>