net: dsa: mv88e6xxx: add RMU disable op

The RMU mode bits moved a lot within the Global Control 2 register of
the Marvell switch families. Add an .rmu_disable op to support at least
3 known alternatives.

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>