net: dsa: mv88e6xxx: Reserved Management frames to CPU

Older devices have a couple of registers in global2. The mv88e6390
family has a single register in global1 behind which hides similar
configuration. Implement and op for this.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>